The transaction needs to be in the same currency as the A/R and A/P accounts you use.

You must place USD orders under a USD customer and use a USD A/R account

Add A/R account rule

If you sync sales receipts, this rule is unnecessary. To change the A/R based on the currency code field, follow these steps:

1. Click rules > my rules on the left.
2. Click add new rule.
3. Select map field, based on a condition.
4. As the action, enter map order A/R account.
5. Under what conditions, enter these values: order currency code, equals, CAD.
6. Under what should the value be, enter Accounts Receivable (A/R) - CAD.
7. Report steps 2 - 6 for each rule.