How is the repayment of a debt for which repayment is not expected zakat-eligible, if each installment does not reach the the minimum threshold (nisab)?

The debt owed by an insolvent person is subject to zakat (definition) for one year after its recovery. If the creditor recovers a nisab (definition) (the minimum amount liable for zakat), they must pay zakat on it. If they recover less than a nisab, no zakat is due, unless they had previously recovered a nisab. So, if they recover less than a nisab and then recover an amount that completes the nisab, the hawl (zakat year) for the initially recovered amount begins from the time the nisab is completed, and both amounts are then subject to zakat together. Any amount recovered after the completion of the nisab is subject to zakat, even if it is small.