Thanks for joining the thread, Carrera09s.

I know how important adding an existing invoice to a project is. However, it will remove linked transactions from an invoice. This is why we don’t recommend adding invoices linked to more than one other transaction. You can delete the invoice first and link the Quote to the project as a workaround. You can utilize the steps provided by my peer above.

While this feature is unavailable, I recommend sending feedback to our product developers. This way, they can further review which parts of the program need enhancements and consider adding them to future updates. 

Please know that not all requests will be implemented immediately. With that, we cannot provide a specific timeframe as to when this will roll out. However, you can visit the Customer Feedback page to get updates about the status of your request.

For future reference, here's an article to learn how to track invoice payments: Record invoice payments in QuickBooks Online.

We appreciate your patience on this. Please, feel free to let me know if you require any further information by leaving a reply below. Take care.