What should I do with the amount my friend will pay me, knowing that I don't remember the people who shared with me in paying part of it to him, and we all paid it on the basis that we don't want it back?

A gift is not complete without possession. If the donee has not taken possession of the gift, then the money still belongs to its original owners. If it is possible to deliver it to them, then do so. Otherwise, give it as charity on their behalf. If they come later, they will have the choice between approving the charity given on their behalf or taking an equivalent amount. It was necessary to inform the owners of the money that the man did not accept the gift and to consult with them about giving him the amount as a loan, documented in writing.