Is it permissible to eat from supermarket goods in exchange for a dedicated food allowance provided for purchasing books or clothes?

As long as you are eating from the shop's goods without the owner's knowledge and permission, despite knowing that he allocated money for you to break your fast outside the shop, then this eating is not permissible. This is based on the Prophet's (peace be upon him) saying: "The wealth of a Muslim person is not lawful except with his willing consent." You must seek the shop owner's forgiveness or return the value of what you ate to him. As for the books and clothes that you bought with the money the shop owner allocated for breaking fast, there is no harm in you benefiting from them.