How is Zakat to be paid when selling a plot of land inherited for the purpose of settling accumulated debts from a bank loan and family debts, knowing that the sale price does not cover the entire debt?

There is no Zakat on land sold out of necessity and not for trade. Zakat is due on its price if it reaches the [nisab] threshold and a full year has passed since its possession. If debt encompasses your wealth, then there is no Zakat upon you.