Final answer:
If the contract requires payment by a certain deadline and the other party fulfills that requirement, they are not in breach. By refusing the check, you would be in breach, not Jen.
Step-by-step explanation:
According to contract law, if a contract specifies a deadline for payment and the other party fails to meet that deadline, they are considered to be in breach of contract. In this case, since the contract requires Jen to pay by 5 p.m. Sunday and she gives you a check at that time, she has fulfilled her contractual obligation. Therefore, if you refuse her check, it would be considered a breach of the contract on your part, not on Jen's.