We have 144 tenants & their invoices are set up to recur monthly in USD . The amount we receive for a US$ is different to the market rate .

Welcome to the QuickBooks Community, Ldecaul. Let me discuss how Multicurrency works in QuickBooks. Then, ensure you can send feedback so your suggestion is forwarded to the Product Development Team. 

When you turn on multicurrency, you can use exchange rates from IHS Markit or your own exchange rates for foreign currency transactions. Then, if you want to use your own rate for multiple transactions, you have the option to update the rate in your currencies list.

However, you can only use the exchange rate for the whole day and you'll have to reenter the rate if you want to use it again for a future date. That said, a way to put in a custom rate for a date in advance is currently unavailable.

I encourage you to submit feedback about having an option to assign a custom rate in advance date. These requests and suggestions are forwarded to the Product Development Team for consideration in future updates. To do this, you'll have to go to the Gear icon and choose Feedback.

Lastly, you may refer to this article to see various details on how QuickBooks uses exchange rates for foreign currency transactions and how it works: Learn about exchange rates in QuickBooks Online.

I'll be around to help if you have any other questions about managing entries with multi-currency in QBO. Just let me know by leaving a comment below. Have a nice day ahead, Ldecaul.