Is it permissible for a merchant to deduct one thousand dollars from a forty-thousand-dollar debt owed by his customer, in exchange for the customer expediting the payment of ten thousand dollars in cash from this debt?

The aforementioned transaction, which is the waiving of part of a debt in exchange for prompt payment of another part, is impermissible according to the majority of the four Imams. This is because it is usury (riba) and is known by the rule "give up and hasten" (ضع وتعجل). A number of Companions and their successors (Tabi'in) prohibited this.