What is the ruling on paying off a debt that its owner sold to a collection company? Is one absolved of the debt by that sale, or by paying the company?

According to the majority of jurists, it is not permissible to sell a debt to anyone other than the debtor due to the inability to deliver and the presence of gharar (definition) (excessive uncertainty), and because it could be an interest-based contract. Furthermore, discounting commercial papers is not permissible in Shari'ah. The fundamental principle is that you must repay the full debt to its owner. Your obligation is not discharged merely by the creditor stating that he will not demand it from you, as his statement was based on a mistaken assumption. If the creditor assigns the company to collect from you, then you are obligated to repay the principal amount of the debt to the company, if possible, to avoid legal pursuit. If the company insists on the full amount including its usurious increase, you are not obligated to pay this unless you fear the harm of legal pursuit. If it is impossible to repay the debt, then it is permissible to give charity on behalf of the rightful owner.