While this can't be done from Adagio, or any other accounting software that I'm aware o as this effectively contravenes GAAP, there is nothing to stop your client from revaluing AR & GL afterwards (issue the invoices at 2.7 and the revalue to 2.6882). This would book the necessary foreign exchange adjustment, something that I suspect your client doesn't want to see.

Another option would be to issue the invoice at 2.6882 but then use PrintBoss to change the math on the printed document (invoice).

What does your client expect to accomplish from this exercise? At the end of the day if your client receives payment at 2.7, it's in AR at 2.6882.