How to update budgets with a specific currency exchange rate

We had a project which was funded by an agreement using the USD currency.   When the funding was baselined, it generated a baselined revenue budget on the project in the CAD currency (because the project currency is CAD) using the exchange rate defined for the baseline date.   Months later, we decided that we wanted to zero out the revenue budget and the funding because the project was cancelled.   We need the credits to be entered using the same exchange rate so that both the CAD revenue budget and USD agreement funding will be zeroed out.

How should we accomplish this?   Should we update the revenue budget to be $0 CAD and if so, how do we specify the exchange date or the exchange rate so that the project funding will be credited to $0 USD by using the same exchange rate used for the original budget (we are managing our budgets as financial plans in the self-service forms)?   Or should we update the project funding to be $0 USD and if so, how do we specify the exchange date or the exchange rate so that the project revenue budget will be credited to $0 CAD?