Is it permissible for the collector to take dollars from subscribers, then exchange them for riyals, pay the company the value of the bills in riyals according to the company's fixed rate, and keep the surplus difference for himself, especially when the dollar exchange rate in the market is higher than the company's fixed exchange rate?

Electricians, agents, and representatives are entrusted with the money in their possession, and it is not permissible for them to dispose of it except with the owner's permission, based on the words of Allah Almighty: (Indeed, Allah commands you to render trusts to whom they are due) An-Nisa/58. They must hand over the money to the company as they received it from the subscribers, and it is not permissible for them to change it from one currency to another. Any increase resulting from this belongs to the company, and they are not allowed to take anything from it.