Is it permissible for the borrower from the "Association" to pay the one thousand Dirhams that another person paid on behalf of the lender, or to buy a gift for the lender with it?

Your friend is a lender to you, and it is not permissible for you to promise him an increase of a thousand on the loan amount, as that would be usury (riba). However, if you wish to honor him, there is no harm in giving him the extra thousand when returning the amount, out of good repayment, without a prior promise. The Sharia has permitted the borrower to give an increase upon repayment without a prior agreement, as an act of kindness and benevolence, based on the Prophet Muhammad's (peace be upon him) saying: "The best among you are those who are best in repaying their debts." As for a gift from the one who owes the debt, it is preferable to be cautious if it is known that its reason is the loan, unless it is known that it is for kinship or friendship, or if the borrower is known for generosity.