If the owner agrees, it discharges the debt under accord and satisfaction, as long as he takes the funds (cashes the check) knowing or with reason to know that it's consideration for discharging the debt. Even though it seems there's no new consideration here, there actually is; because it's a new promise to pay, given the fact that the owner would probably not be able to collect any funds otherwise.

As for the interest, if Mrs. Green disagrees with the calculation, she can recover under contract theory if she is proven correct. Unilateral mistake can be corrected in this fashion. If however, she knew of the error, then she would not be able to collect after the fact. Of course, she has to prove in a court that the calculation was incorrect.

A contract must have offer, acceptance and consideration (i.e., some forebearance) and this contract had all three. Paul offered a severance package, which Mary accepted by cashing the check and agreeing, and the consideration was her giving up her right to sue.
Harry reasonably relied to his detriment on Paul's comment that he would sell him the trains, or at least give him right of first refusal. Based on this belief, Harry acted reasonably in building more room for the trains and borrowing the money to do so. Granted, there is no true contract, but there is promissory estoppel under reliance theory. Harry is entitled to buy the trains, or recover for his reasonable expenses in preparing for the trains' arrival.