Re: counter offers
It
is confusing, but mainly because of the manner in which you chose to express the problem.
An answer to your problem is hard to determine because you have not given us greater insight into the communication that occured between the parties. Let's reconsider it in three different ways with star actors Jim and Bob:
Jim: If you continue to work for me, and then work one additional day, I will give you X.
Bob: I agree to that, but only if I get overtime for the extra day.
Jim: Accepted
In this case the offer has been remade by Bob on Jim's original terms,
plus the extra provision for overtime which he has decided to include. I would say that were the correspondance similar to what has occured here, then the terms of the contract allow for X plus the overtime requested by Bob/your plaintiff.
Jim: If you continue to work for me, and then work one additional day, I will give you X.
Bob: Give me overtime for the extra day and I'll do it.
Jim: This is a clear an unequivocal acceptance of your counter offer.
In this case it becomes a bit more ambiguous, and so a little harder to discern how a court would interpret the matter. However, I would say that Bob's counter offer, in contrast to the previous example, has a limited scope here - he is merely requesting overtime for the additional day, and appears to have completely disregarded the offer of X. Thus, in this case, I would say that the terms of the contract made provision for the overtime, and nothing more.
Jim: If you continue to work for me, and then work one additional day, I will give you X.
Bob: This is a counter offer.
Jim: Fuck you, I'll outsource it.
This should be pretty obvious. The example you gave did not state how, or indeed if, the original offeror accepted his counter offer. Further negotiation/no acceptance = no contract. I'm pretty positive you'll have the relevant authority for this in your casebook.