Is it permissible to subtract the value of a debt in euros from another debt in riyals, with the remainder being paid based on today's exchange rate?

"Set-off" (al-muqassah) is the discharge of a confirmed debt owed by one party against a confirmed debt owed by the other, and it is one of the methods for settling debts. If the debts are identical in all aspects, they are automatically set off. If they differ in amount or maturity, set-off is permissible by mutual agreement. If the debts differ in type (e.g., gold for silver), scholars have differed regarding the ruling on set-off: the Shafi'is and Hanbalis prohibited it, while the Malikis permitted it on condition that both debts are immediately due, and the Hanafis permitted it absolutely.

The preferred view is the permissibility of set-off between two debts even if they differ in type, because it is a recovery of rights and not a sale, and recovery has fewer conditions than a sale.