Hi Karen
Purchase Orders for Adagio, which integrates with both Adagio AP and Adagio JC, has this feature. I have sold it to one client strictly for this one feature! It actually alerts you not when you enter the invoice, but when you issue the PO, which is even better. If you don't issue you a PO, you can enter the invoice directly as a "quick receipt", and I believe it will alert you there as well.
Now that I wrote all of that, I am hoping it is correct, because in truth it was Inventory Receipts (the DOS version of POA) that I sold in this case. Check with the folks at The Stief Group to confirm that it is also true for POA (www.stiefgroup.com)
Regards
Steve Schwartz
steve@sschwartcpa.com