You need to make your a second relationship based on a calculated field.
Use a text field named Sold in the portal table for the omit and place it on the portal line formatted as a checkbox with a value list assigned with the one value, 'Sold". When you check it, it highlights as x or uncheck it and it is empty.
The calculated field then is Sold & " " & LinkIdField.
This will produce:
Sold 1234
1234
Link the calculated field of the portal table to the LinkIDField of the master table. When Sold is checked there will be no matching value in the master table and the portal line will dissappear.
You would need two layouts, one for creating portal lines and then one for using the Sold concept. With the newer Filemakers, you would create a tab layout with New and Available tabs.