When the adjustment is created, you choose the Adjusted document number that you want to apply it to. If the document is in the current file, it will be displayed in the finder for this field.

From this example, my guess is that the Adjusted document number was not entered correctly. If it had been, the Invoice and the Adjustment would have a current balance of 0.00:
69 Nationial Bank of Canada
IN 21-1 99/02/22 150.00 150.00
AD 21-1 99/02/25 -150.00 -150.00

If the document number entered as the Adjusted document number was not on file, the Adjustment will have a current balance equal to the original balance.

To match the adjustment to an invoice on file, you can create a manual check for $0.00 and choose to pay the two transactions. Once the current amounts are reduced to 0, the next period end processing will move the transactions to History.