He was asked:

A woman lent her husband money, then said to him, “I am afraid you will not repay me.” He told her, “If I do not repay you by the end of this Ramadan, you are divorced three times.” The husband is away in Qūs and has appointed no one to act on his behalf. If the woman releases her husband from the debt and the month passes, does the divorce take effect or not? If someone voluntarily pays the debt on his behalf, is the debt discharged without the divorce taking effect when the month passes, or does it take effect?

He answered:

If she releases him from the debt, he does not break his oath according to many jurists, including Abū Hanīfa and Muhammad, and according to one opinion within Ahmad’s school and elsewhere. This is for two reasons:

First: releasing him from the debt precludes repayment, making its payment impossible.

Second: an act that one swears to perform is comparable to an act that one is commanded to perform. It is known that a person is commanded to repay a debt only so long as it remains outstanding. The same applies to oaths and customary usage; the two cases are analogous. Ordinarily, someone who swears such an oath intends to free himself of liability, put an end to the creditor’s demands against him, and repay the creditor if the debt remains outstanding.

Likewise, if someone pays the debt on his behalf, his liability for it is discharged without any action on his part, just as it is discharged when the debt is remitted. Repayment on his part becomes impossible, and the creditor’s purpose has been fulfilled. The Prophet—may Allah bless him and grant him peace—treated payment on a debtor’s behalf as equivalent to the debtor’s own payment when he said:

“Tell me, if your father had…”

And in another hadith:

“Your mother had a debt, and you repaid it on her behalf, would that discharge the obligation?” She said, “Yes.” He said, “Allah has a greater right to be paid what is due.”

And Allah knows best.