Hi C.O.W.
If by total luck (I realize the invoice is years old, but you never know) you have the check reconciliation feature in Adagio turned on, but you don't use it (i.e. the check is still showing in the system as outstanding), then you can clean up the offending transaction.
Under Payments | Check Reconciliation you will see a list of checks issued. If the check is still there and it has a status of "O", then change the status to "R" for reversed. Get out of Payables and then back in, but this time when you enter your user ID and password, change the date to be in 2096. Then go back to Payments | Check Reconciliation and post the check reconciliation. You will then see offsetting transactions in the vendor's account from that date. You are back to square one.
Now, exit Payables and re-enter with today's date. Re-enter the check in a manual check batch with the correct date, applying it to the original invoice(s), which should now show as unpaid. No need to print the check. Post the batch. Two offsetting transactions should be waiting to go to the GL.
There is no other way to do this without resorting to tech support's suggestion.
Steve