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>The PK is really assigned in the default value of the field in the table


If that is true, you shouldn't need this line in your ADD/SAVE button:

REPLACE PAID_ID WITH GETKEY('PAID_TOB')

If you put GETKEY('PAID_TOB') as the default value for the PAID_ID field, it will automatically populate the PAID_ID field when you do an append on that table. There is no need to have the above line (PAID_ID will be assigned a new value again).