Is it considered forbidden to alter the exchange rate in invoices to be lower than the agreed-upon rate (the central bank exchange rate for the first day of the month) in order to save money for the company, while the rate is corrected if it is higher than the agreed-upon rate?

Cheating is not permissible. Contracts must be fulfilled, and the agreed-upon conditions for setting the exchange rate must be adhered to, based on Allah’s Almighty saying: "O you who have believed, fulfill [all] contracts," and His saying: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]," and the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions," and "Whoever cheats us is not of us, and deception and treachery are in the Fire."