This solution will allow you to create a open stock item that will provide a credit/negative amount.
How it works:
1. When button is pressed it will display the Sell Price screen 
2. Once amount is entered it will display as a negative amount 
Setup:
1. Go to Setup > Global Options > Sales
2. Tick Non-Turnover Stock Items
3. Create a Department for the payout items to report under
4. Tick 'Non-Turnover'
5. Save & Exit
6. Create Payout Stock Item(s)
7. Tick Force Selling Price Entry or set payout Amount
8. Go to Setup > Keyboard Program
9. Create a Keyboard shortcut with a function of X (Quantity)
10. Enter -1 in the automatic text 
11. Create another keyboard function type of Stock item
12. Enter the Stock Item code into the Automatic Text (eg 9990001) 
13. Create a Macro Key on the POS Screen
14. Enter the Keyboard Shortcuts used in steps 7 - 10 eg {F4}{F5}