Is it permissible for a man who found money in his house belonging to a woman who visited him twenty years ago, and whose whereabouts he no longer knows, to donate it to the poor?

A man's denial of finding money in his house, knowing it fell from a female visitor, is a form of theft, which is a major sin. He must repent to Allah, and the completeness of his repentance is to return the money to its owner. If he is unable to do so and cannot find her, he should give it as charity on her behalf. If he later finds the owner of the money or her heirs, he should give them the choice: either they ratify the charity and its reward is theirs, or he gives them an equivalent amount to what he donated, and in that case, the reward for the charity will be his.