Is it permissible to take money from a person with the agreement not to return it, knowing that the source of this money is unknown?

Ignorance of the source of funds received, whether a gift or a loan, does not cause harm. There is no obligation to return a gift if the giver is not legally incapacitated. As for a loan, it must be repaid immediately by the solvent debtor upon demand. If the debtor is in hardship, the creditor should grant him respite until Allah eases his situation.