Reuven took Shimon to secular court without a heter from beis din. Reuven won a monetary claim to be paid out over time. Reuven now offers Shimon a discount to pay early.
May Shimon agree to this arrangement, since it saves him money that Reuven wasnt entitled to, or does the issur of ribis still apply?
It is fine for Shimon to agree to the arrangement.
If we assume that Shimon actually owes the sum that the court ruled on, then the debt is extant, and there is no problem in paying less. And if Shimon doesn’t owe the money, then the entire payment is gezel (theft), and therefore there’s definitely no problem of paying less.
Even for a store, the problem of ribis arises when there are two prices, one for paying in installments, and one for paying up front. The reason for this is that peyment in installments would therefore be ribis: the customer is being charged extra for the “time” of paying in intstallments.
However, if the “real” price is the more expensive price, and a discount is given for paying up front, the problem of ribis does not arise.