How does an exception (Istithna) operate in an admission of debt compared to a subsequent declaration of payment?

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An exception operates by defining the remaining balance of the originally stated amount. For example, the statement like the verse {He remained among them a thousand years except for fifty years} (Quran 29:14) is interpreted as nine hundred and fifty years. In contrast, payment acts by removing an established portion of the debt; once removed by payment, it is impermissible to describe the situation using terminology that implies the debt still exists in its entirety or in the manner claimed as paid.