This forum is to discuss different features/issues of Oracle Financials modules ( GL - General Ledger, AP - Accounts Payable, AR - Accounts Receivable, FA - Fixed Assets & CM - Cash Management ).
Refund is the only payment type that can have a negative amount (11.5.10.2).
It is a seeded lookup type with System Access Level. That's probably why you cannot override it with other payment type.