Answer: O Tim must pay because the agreement to complete the tax returns earlier than originally agreed is additional consideration supporting the modification of the contract.
Step-by-step explanation:
When Tim agreed to Betty's stipulation that for her to finish the returns earlier, he would have to pay an extra $250, he in effect agreed to the modification of the contract.
Modified Contracts are also enforceable by law so Tim has to pay the $250. There was no proof that Betty acted wrongfully as she had to change her schedule and needed to be compensated for the inconvenience. Also even if the modification was not in writing, it is a generally accepted rule that for contracts to be modified orally, the amount must not exceed $500 which it did not.
Tim is very much liable to pay.