A check is not a contract. It is a negotiable instrument that is governed by your state’s version of the Uniform Commercial Code. Your agreement, whether oral or written, is a separate matter.
The check you have is now considered “stale-dated.” This means that the bank that it is drawn from has the option to either accept or bounce the check. Essentially, there is a risk if you deposit this check that it may be returned and there could be fees imposed.
As usual, you are best trying to resolve this matter amicably. You may want to offer to pay the fees for a stop-check when they provide a money order or similar good funds. Good luck.