What is the ruling on disposing of a remaining amount from the sale of a house that was purchased with an interest-based loan after paying off the principal of the loan? Is it permissible to use it personally, give it to relatives, contribute it to mosque maintenance, or make it an endowment?

Usury is forbidden and a major sin, but its prohibition pertains to thebler of the contract, not to the money itself or what was purchased with it. The house that the questioner bought with this loan is his property, and he is permitted to dispose of it. The questioner is obligated to repent from the sin of usurious borrowing. If he is able to pay back only the principal amount, he must do so; otherwise, the sin falls upon the lending institution.