You certainly have caused a mess.   And the fact that you are asking whether you are liable suggests that you think you might be, or at least that you ought to be.

Whether you are legally liable may depend on a very obscure provision of your state law called the “statute of frauds.”  Under this concept, certain contracts must be in writing to be enforceable.  The statute of frauds often requires a written agreement to sell real estate or transfer an interest in real estate.  Whether it would cover a four-day license to occupy your house is something I can’t answer from here.

Unless you are off the hook legally, the fact that it was an oral agreement would not normally prohibit enforcement.  But I assume that whatever the law may be, the bid price is too small to warrant any kind of legal proceedings.  Therefore, if you don’t want to mess up your reputation as well as the charity’s, you may want to make things right.

You ought to be talking to the charity and the couple.  If the couple really wants to use your house, you could go away for a couple of days and let them use it.  Maybe they would be satisfied with a weekend instead of four days. If the couple really doesn’t care that much, you could give the charity the bid price instead of the use of the house and the charity could give the bid back to the couple.  Maybe you could give a few dollars extra to the couple, so everybody thinks they win.  There are a lot of permutations that could conceivably resolve this issue without leaving you looking like an irresponsible jerk.  But you have to talk to them to find out.