You have two transactions and you have to account for full deposit into your account first. You cannot create a net deposit that also has expenditure of 80% out and 20% retained and still show the money being deposited.
Handle the deposit first. Is it income? Is it a passthrough? A passthrough account wouldl be a current liability when deposited. Then you can record a check that is set up the following. line 1 is 100% of collected funds owed and line 2 is a negative 20% of line 1. Line 2 will be an income account. End result you collect 100% of money on behalf of someone else and have recorded 20% of it as your fee for handling.
