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17.3 Sales Law Compared with Common-Law Contracts and the CISG

Learning Objective

  1. Recognize the differences and similarities among the Uniform Commercial Code (UCC), common-law contracts, and the CISG as related to the following contract issues:

    • Offer and acceptance
    • Revocability
    • Consideration
    • The requirement of a writing and contractual interpretation (form and meaning)

Sales law deals with the sale of goods. Sales law is a special type of contract law, but the common law informs much of Article 2 of the UCC—with some differences, however. Some of the similarities and differences were discussed in previous chapters that covered common-law contracts, but a review here is appropriate, and we can refer briefly to the CISG’s treatment of similar issues.

Mutual Assent: Offer and Acceptance

Definiteness of the Offer

The common law requires more definiteness than the UCC. Under the UCC, a contractual obligation may arise even if the agreement has open termsUnder the UCC, a term that has not been expressed in the contract.. Under Section 2-204(3), such an agreement for sale is not voidable for indefiniteness, as in the common law, if the parties have intended to make a contract and the court can find a reasonably certain basis for giving an appropriate remedy. Perhaps the most important example is the open price term.

The open price term is covered in detail in Section 2-305. At common law, a contract that fails to specify price or a means of accurately ascertaining price will almost always fail. This is not so under the UCC provision regarding open price terms. If the contract says nothing about price, or if it permits the parties to agree on price but they fail to agree, or if it delegates the power to fix price to a third person who fails to do so, then Section 2-305(1) “plugs” the open term and decrees that the price to be awarded is a “reasonable price at the time for delivery.” When one party is permitted to fix the price, Section 2-305(2) requires that it be fixed in good faith. However, if the parties intend not to be bound unless the price is first fixed or agreed on, and it is not fixed or agreed on, then no contract results.Uniform Commercial Code, Section 2-305(4).

Another illustration of the open term is in regard to particulars of performance. Section 2-311(1) provides that a contract for sale of goods is not invalid just because it leaves to one of the parties the power to specify a particular means of performing. However, “any such specification must be made in good faith and within limits set by commercial reasonableness.” (Performance will be covered in greater detail in Chapter 18 "Title and Risk of Loss".)

The CISG (Article 14) provides the following: “A proposal for concluding a contract addressed to one or more specific persons constitutes an offer if it is sufficiently definite and indicates the intention of the offeror to be bound in case of acceptance. A proposal is sufficiently definite if it indicates the goods and expressly or implicitly fixes or makes provision for determining the quantity and the price.”

Acceptance Varying from Offer: Battle of the Forms

The concepts of offer and acceptance are basic to any agreement, but the UCC makes a change from the common law in its treatment of an acceptance that varies from the offer (this was discussed in Chapter 8 "Introduction to Contract Law"). At common law, where the “mirror image rule” reigns, if the acceptance differs from the offer, no contract results. If that were the rule for sales contracts, with the pervasive use of form contracts—where each side’s form tends to favor that side—it would be very problematic.

Section 2-207 of the UCC attempts to resolve this “battle of the forms” by providing that additional terms or conditions in an acceptance operate as such unless the acceptance is conditioned on the offeror’s consent to the new or different terms. The new terms are construed as offers but are automatically incorporated in any contract between merchants for the sale of goods unless “(a) the offer expressly limits acceptance to the terms of the offer; (b) [the terms] materially alter it; or (c) notification of objection to them has already been given or is given within a reasonable time after notice of them is received.” In any case, Section 2-207 goes on like this: “Conduct by both parties which recognizes the existence of a contract is sufficient to establish a contract for sale although the writings of the parties do not otherwise establish a contract. In such case the terms of the particular contract consist of those terms on which the writings of the parties agree, together with any supplementary terms incorporated under any other provisions of this Act.”This section of the UCC is one of the most confusing and fiercely litigated sections; Professor Grant Gilmore once called it a “miserable, bungled, patched-up job” and “arguably the greatest statutory mess of all time.” Mark E. Roszkowski, “Symposium on Revised Article 2 of the Uniform Commercial Code—Section-by-Section Analysis,” SMU Law Review 54 (Spring 2001): 927, 932, quoting Professor Grant Gilmore to Professor Robert Summers, Cornell University School of Law, September 10, 1980, in Teaching Materials on Commercial and Consumer Law, ed. Richard E. Speidel, Robert S Summers, and James J White, 3rd ed. (St. Paul, MN: West. 1981), pp. 54–55. In 2003 the UCC revisioners presented an amendment to this section in an attempt to fix Section 2-207, but no state has adopted this section’s revision. See Commercial Law, “UCC Legislative Update,” March 2, 2010, accessed March 1, 2011,

As to international contracts, the CISG says this about an acceptance that varies from the offer (Article 19), and it’s pretty much the same as the UCC:

(1) A reply to an offer which purports to be an acceptance but contains additions, limitations or other modifications is a rejection of the offer and constitutes a counter-offer.

(2) However, a reply to an offer which purports to be an acceptance but contains additional or different terms which do not materially alter the terms of the offer constitutes an acceptance, unless the offeror, without undue delay, objects orally to the discrepancy or dispatches a notice to that effect. If he does not so object, the terms of the contract are the terms of the offer with the modifications contained in the acceptance.

(3) Additional or different terms relating, among other things, to the price, payment, quality and quantity of the goods, place and time of delivery, extent of one party’s liability to the other or the settlement of disputes are considered to alter the terms of the offer materially.

Revocation of Offer

Under both common law and the UCC, an offer can be revoked at any time prior to acceptance unless the offeror has given the offeree an option (supported by consideration); under the UCC, an offer can be revoked at any time prior to acceptance unless a merchant gives a “firm offerA promise by a merchant to buy or sell goods in a signed (or “authenticated”) writing that is not revocable during the time stated or for a reasonable time not to exceed six months (UCC 2-205).” (for which no consideration is needed). The CISG (Article 17) provides that an offer is revocable before it is accepted unless, however, “it indicates…that it is irrevocable” or if the offeree reasonably relied on its irrevocability.

Reality of Consent

There is no particular difference between the common law and the UCC on issues of duress, misrepresentation, undue influence, or mistake. As for international sales contracts, the CISG provides (Article 4(a)) that it “governs only the formation of the contract of sale and the rights and obligations of the seller and the buyer arising from such a contract and is not concerned with the validity of the contract or of any of its provisions.”



The UCC requires no consideration for modification of a sales contract made in good faith; at common law, consideration is required to modify a contract.Uniform Commercial Code, Section 2-209(1). The UCC requires no consideration if one party wants to forgive another’s breach by written waiver or renunciation signed and delivered by the aggrieved party; under common law, consideration is required to discharge a breaching party.Uniform Commercial Code, Section 1–107. The UCC requires no consideration for a “firm offer”—a writing signed by a merchant promising to hold an offer open for some period of time; at common law an option requires consideration. (Note, however, the person can give an option under either common law or the code.)

Under the CISG (Article 29), “A contract may be modified or terminated by the mere agreement of the parties.” No consideration is needed.

Form and Meaning

Requirement of a Writing

The common law has a Statute of Frauds, and so does the UCC. It requires a writing to enforce a contract for the sale of goods worth $500 or more, with some exceptions, as discussed in Chapter 13 "Form and Meaning".Proposed amendments by UCC revisioners presented in 2003 would have raised the amount of money—to take into account inflation since the mid-fifties—to $5,000, but no state has yet adopted this amendment; Uniform Commercial Code, Section 2-201.

The CISG provides (Article 11), “A contract of sale need not be concluded in or evidenced by writing and is not subject to any other requirement as to form. It may be proved by any means, including witnesses.” But Article 29 provides, “A contract in writing which contains a provision requiring any modification or termination by agreement to be in writing may not be otherwise modified or terminated by agreement.”

Parol Evidence

Section 2-202 of the UCC provides pretty much the same as the common law: if the parties have a writing intended to be their final agreement, it “may not be contradicted by evidence of any prior agreement or of a contemporaneous oral agreement.” However, it may be explained by “course of dealing or usage of trade or by course of performance” and “by evidence of consistent additional terms.”

The CISG provides (Article 8) the following: “In determining the intent of a party or the understanding a reasonable person would have had, due consideration is to be given to all relevant circumstances of the case including the negotiations, any practices which the parties have established between themselves, usages and any subsequent conduct of the parties.”

Key Takeaway

The UCC modernizes and simplifies some common-law strictures. Under the UCC, the mirror image rule is abolished: an acceptance may sometimes differ from the offer, and the UCC can “plug” open terms in many cases. No consideration is required under the UCC to modify or terminate a contract or for a merchant’s “firm offer,” which makes the offer irrevocable according to its terms. The UCC has a Statute of Frauds analogous to the common law, and its parol evidence rule is similar as well. The CISG compares fairly closely to the UCC.


  1. Why does the UCC change the common-law mirror image rule, and how?
  2. What is meant by “open terms,” and how does the UCC handle them?
  3. The requirement for consideration is relaxed under the UCC compared with common law. In what circumstances is no consideration necessary under the UCC?
  4. On issues so far discussed, is the CISG more aligned with the common law or with the UCC? Explain your answer.