How do you pay Zakat on money while you are in debt, and does debt prevent Zakat?

Cash and trade goods are evaluated and added together, then the debt is deducted. If a nisab remains and a hawl has passed on it, zakat becomes obligatory; otherwise, it does not. The nisab is the equivalent of 85 grams of gold or 595 grams of silver, and the obligatory amount is a quarter of a tenth (2.5%). If there is surplus money beyond basic needs, it should be allocated to cover the debt, so that the zakat-eligible wealth is not diminished.