One of my customers' ERP system is a nightmare so they outsource many purchases to a reseller. I didn't understand what was going on when the reseller contacted

One of my customers' ERP system is a nightmare so they outsource many purchases to a reseller. I didn't understand what was going on when the reseller contacted me, and ended up being added as a supplier to my customer's ERP. I then got paid twice. After 9 months and multiple inquiries I still don't know how to give them their money back!