If it works for you then great! You are basically adding 500 to the number of rows in the returns table.

It may go a bit wonky when you start deleting them - I would do a bit of testing when you complete a RMA and delete the row from the dbase (although it may not delete them - in which case you should be fine.)

Did you try the other method I posted? If that works I would use it as you are changing less of the core code of the contribution ... ie. you are only displaying the returns_id rather than the rma_value to the admin user ...