Hi there, rposada.
Thanks for taking the time to visit the Community. I'm here to provide some information on your question about turning off the exchange rate at payment when using multicurrency.
According to a QuickBooks Online guide, "Creating multicurrency transactions is like creating regular transactions. Every transaction
includes an exchange rate which can be edited. However, the exchange rate should be current and editing is not required."
It is possible for the exchange rate to be edited but it can't be turned off. The guide I have for you is labeled for Canada but I think you'll still find it helpful. You can check it out here: http://intuitglobal.intuit.com/delivery/cms/prod/sites/default/education.intuit.ca/downloads/Appendi.... Take a look at page 3 for more information about exchange rates.
I wish you and your business continued success. Please let me know if you have any other questions.