Sage 50 - what is "Link Removed"

Any Sage accounting gurus here that can help? Looking at this screen clipping from Sage should explain my issue.

This customer has two transactions in the activity - a sales invoice for £2,085.62 and a sales receipt of £6,677.23. I am trying to find out which customer the balance of £4,591.61 went to?

This customer does not show up as having an outstanding amount on the Aged Debtors report, and the total of the Aged Debtors report agrees to the balance on the debtors control account in the nominal ledger.

Does anyone know how the "Link Removed" situation is likely to have come into existence?