What is the ruling on waiving the remainder of an amount owed or surplus to the seller or debtor?

There is no objection to combining exchange and gift, or sale and gift. So, if you give a seller ten riyals for an item that costs nine, and the tenth riyal is a gift, or you leave it with him as a trust, there is no harm in that.

What is forbidden is to exchange ten riyals for ten, where he gives you nine and defers the tenth; this is prohibited usury (riba).

Jurists have explicitly stated the permissibility of the remaining amount paid being held as a trust by the seller, or used to purchase something, or as a down payment (salam), or for the buyer to gift it to the seller.

Therefore, there is no suspicion of usury in the buyer leaving a portion of his money with the seller, because it is a sale and entrustment, not an exchange.