Go to Process...Match Open Item, and select the relevant customer code.

You will then see a listing of all the invoices.

The invoices that have been matched against receipts will have a little + sign on the left to it. Click on the + sign to expand the breakdown of the original invoice and the receipt/s linked to it.

Any receipts not yet allocated with be grouped under the Unallocated line item at the top of the list.

In order to reallocate receipts, simply drag the receipt line items from either the Unallocated section or from existing invoice lines and drop them on top of the relevant invoice lines.