When does debt prevent the obligation of Zakat?

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Debt prevents the obligation of Zakat if it either completely exhausts the Nisab (minimum threshold) or reduces the remaining property below the Nisab, provided the debtor possesses nothing else from which to repay the debt other than the Nisab amount itself or what is indispensable. For instance, if one possesses twenty mithqals and owes one mithqal or more or less such that paying the debt reduces the remaining amount below the Nisab, and there is no other means of repayment outside the Nisab, Zakat is withheld. If one has thirty mithqals and owes ten, Zakat is due on the remaining twenty. If one owes more than ten, no Zakat is due. If one owes five, Zakat is due on twenty-five. Similarly, if one has one hundred sheep and owes the equivalent of sixty, Zakat is due on the remaining forty. If the debt equals sixty-one, Zakat is not due because the debt reduces the amount below the Nisab.