Unlinked Receipts are money that was received but not allocated to a specific invoice.

The unlinked receipt can be linked by using a Zero Receipt and moving the money from one line to the correct lines or invoice.

When there are unlinked receipts, the age analysis will be showing incorrectly.  The account will show outstanding amounts as a plus and minus amount on the unlinked receipts that it looks like overpayments on the invoice lines. But the total on the account will display correctly.