He works as a collector for an electricity company. Is it permissible for him to change the currency before giving the collected money to the company?
Question

                A collector who is working in an electricity company is asking the following question: 

The electricity company that he works for considers US$ 100 equal to SR 400 (for example). So whether the bill payer pays US$ 100 or SR 400, it is equal. Sometimes the price of US$ 100 equals SR 403 or more. Is it permissible for the collector to take the bill value in Dollars then exchange them into Riyals and pays 400 Riyals for every US$ 100 he collected and then takes the difference of price for himself?.
Praise be to Allah.Collectors for electricity 
    companies and other representatives are entrusted with whatever money comes 
    to them, and they have no right to dispose of it in any way other than that 
    for which their bosses have given them permission. Allaah says 
    (interpretation of the meaning):
“Verily, Allaah commands 
    that you should render back the trusts to those, to whom they are due”
[al-Nisa’ 4:58]. 
What they must do is give 
    the money to the company as they took it from the customers or subscribers. 
    It is not permissible for them to change it from one currency to another. If 
    they do that, then the additional amount belongs to the company, and they 
    have no right to take anything from it. 
And Allaah knows best.