Is the price difference I get from bringing meals for workers – which I report to the company as one dinar per meal, though I get them for less – lawful or unlawful, given that the company does not pay me for this extra work?

It is not permissible for you to take the price difference because you are an agent for the company, so it is not allowed for you to increase the price at which you buy the food. If this work is not among your duties at the company, you may refuse to do it, or demand additional payment for this work.