a. Generally New Offer
In case a deviation is ascertained, the general rule is laid down in Art. 19(1):
(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.
Obviously, if the purported acceptance does not match the offer there is no meeting of minds and thus no contract conclusion. Yet, the process of negotiation is not cut off there. Rather, a new step is initiated by treating the reply as rejection of the previous offer in combination with a counter-offer containing the newly included alterations. This counter-offer in turn has to be accepted, be it expressly or by conduct.
In practice, it can be difficult to distinguish between a reply purporting to be an offer that includes alterations and an acceptance combined with a new offer to amend the just concluded to contract or conclude a second one. In the latter case a contract is concluded with the terms set down in the original offer and the the original offeror can decide whether it wants to accept the offeree’s return offer.