Is it permissible to sell a debt instrument to an indebted person who is defaulting on payment for half its value?

Selling a debt (sukuk (definition)) to someone other than the debtor for less than its value is impermissible, even according to those who permit the sale of debt. This is because it constitutes usury of delay (riba an-nasi'ah) and usury of surplus (riba al-fadl) if it is for less than its value. The legitimate alternative for recovering a debt is to offer money to whoever facilitates the debt's recovery, whether that be as a wage, a commission, or even a bribe if there is no other recourse, with the sin falling upon the recipient of the bribe.