Not an experienced super cash user!!

If I make a purchase using supercash (say $20 off $50, so I pay $30 for two $25 items....) and return one item -what do I get back? Is the super cash value distributed across all purchases, or if I use $20, do they deduct $20 from my return value (even if I keep some items)?

I want to order joggers in more than one size to try, but if the latter return situation is true, it'd be a pretty bad deal. 😜 TIA!