Is zakat obligatory on a sum of EGP 80,000 remaining from a Murabaha loan, given that the amount of debt owed to the bank is greater than this sum?

This money has become your property and has reached the nisab. If a year passes over it, zakat becomes obligatory on it. You can deduct the debt from it before paying zakat, then pay zakat on the remainder if it reaches the nisab or more. If the debt is greater than the money, then zakat is not obligatory upon you.