Jose,
In MIRO choose "Subsequent Credit" in Transaction box and there you can enter your 0,5 the vendor returned.
Also could you check in table EKBE if fields EKBE-AREWR (or EKBE-AREWB) and EKBE-DMBTR (EKBE-WRBTR) have the same amounts, this should be yoour GR/IR Account.
Kind Regards