«وَلَوْ حَلَفَ لَا يُفَارِقُهُ حَتَّى يَقْضِيَ دَيْنَهُ فَفَارَقَهُ قَبْلَ قَضَاءِ الدَّيْنِ حَنِثَ»
If he swears not to leave another person until the debt is paid and leaves before repayment, he breaks the oath.
Remaining together is a continuing condition, while repayment supplies its explicit endpoint. A future intention to settle the debt does not substitute for remaining until the endpoint occurs.
Thus the particle establishes a measurable boundary: separation before repayment has a different legal consequence from separation after repayment.