Is it permissible to benefit from the difference in exchange rates between the hotel and currency exchange offices when paying hotel expenses during business trips, by exchanging Euros to Francs and paying the bill in the local currency, then submitting the invoice in the expense report as if the payment was made in Euros at the hotel's higher exchange rate?

If the company gives you a lump sum for your travel and accommodation expenses without requiring invoices, then this money is yours. However, if the amount is intended to cover actual expenses, then you are a trustee (of the company's funds), and it is not permissible for you to take more than what you spent. You must inform the company of any profit or reduction you obtain, because the agent's profit belongs to their principal.