All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah.
It is the debtor’s duty to hasten to repay his debt, where it is actually established, and to clear his liability before Allah by giving people their rights. The rights of people are governed by strict accounting, so they are not extinguished except by set-off or by forgiveness. Allah the Exalted says {what means}: “Indeed, Allah commands you to render trusts to whom they are due” [al-Nisāʾ/58]. The Messenger of Allah (peace and blessings be upon him) said: “The hand is responsible for what it has taken until it returns it.” Narrated by Abū Dāwūd and al-Tirmidhī, and al-Tirmidhī said: “This ḥadīth is ḥasan.”
The sum owed by the client to the clinic must therefore be paid to it. If the clinic has closed, its owners must be sought out and their right paid to them, unless they forgive it. The sale of the debt carries no weight, because selling debts for a lesser cash sum is invalid under the Sharīʿah, and an invalid contract has no effect in the Sharīʿah.
If the client cannot find the clinic’s owners after searching for them, he must give that sum in charity for good causes. If the rightful owner later appears, the sum must be paid to him, and the reward of the charity goes to the debtor.
Accordingly, the owners of the clinic must be sought out and the sum paid to them. If reaching them proves impossible, he should give that sum in charity on their behalf for good causes. And Allah Almighty knows best.