What should I do concerning a man who gave me money without my asking or without a word, then demanded it back after a period of time; and am I obligated to return it?

If a person gives money to another without asking or mentioning the term "loan" or anything indicating it, and without any presumption of an intention to lend, then this money is considered a gift (hibah). A gift is concluded by mutual consent and action, unlike a loan which requires an offer or a presumption indicating it. If the giver and the receiver later differ, the absence of mention of a consideration indicates that it is a gift, and the statement of the receiver is accepted, accompanied by his oath, because the apparent situation is with him. If it is proven to be a gift, then it is not permissible to revoke it after its receipt.