The text says وَيُسْتَحْلَفُ الْمُقَرُّ لَهُ فِي الْأَجَلِ: the creditor (muqarr lahu) is asked to swear about the disputed term, not the debtor.
The debtor alleges a right to postpone payment, while the creditor denies that term. Under the rule governing oaths, the denier may be required to swear. The oath addresses whether deferment was agreed, not whether the debt exists at all, since that is admitted.
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Why the creditor is asked to swear
Kitab al-Iqrar: Debt and Trust, Admissions by Reply, and Deferred Debts
The text says وَيُسْتَحْلَفُ الْمُقَرُّ لَهُ فِي الْأَجَلِ: the creditor (muqarr lahu) is asked to swear about the disputed term, not the debtor.
The debtor alleges a right to postpone payment, while the creditor denies that term. Under the rule governing oaths, the denier may be required to swear. The oath addresses whether deferment was agreed, not whether the debt exists at all, since that is admitted.