Case 1: An adult, sane person has wealth below nisab: this rule does not impose zakat.
Case 2: There is nominal nisab, but recognized debt encompasses the entire assessable base: zakat is not due under the stated rule.
Case 3: The net wealth after recognized debt still reaches nisab: the remainder is subject to zakat if the other requirements apply.
Case 4: Zakat is due and the sum is earmarked with intention: that intention can count when it is subsequently paid to a valid recipient.