Is it permissible to take a loan from a bank that buys goods for the client and then sells them to him for a profit margin, on condition that the client authorizes the bank to sell the commodity?

A distinction must be made between a loan and Tawarruq (monetization). A loan that is repaid with a stipulated increase is explicit Riba (usury) and one of the gravest major sins. As for Tawarruq, it is a permissible transaction according to the majority of scholars. If the bank sells a commodity at a deferred price and then authorizes you to sell it without compelling this authorization, then this is permissible Tawarruq.