Do I use the exchange rate on the date of receiving the funds from an invoice or the date at which I raised the invoice?

The CRA requires you to use the Bank of Canada exchange rate in effect on the day you received the income. If the amount was paid at various times in the year, you can use the average annual rate.

https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-re...