Hello Darren,
Follow the below steps:
1. With Tcode ME21N - Create a Returns PO (Activate "Returns" indicator for PO line item)
2. With Tcode MIGO - Carry out Goods Receipt > Purchase Order (movement type selected should be 161)
3. With Tcode MIRO - Create Credit memo w.r.t. Returns PO
Please check and let us know if it works.
Regards,
Naveen K.