I have run into a situation I am not sure how to handle and would appreciate some expert advice. I am not an accountant, just a small business owner, so please keep this as simple as possible.

I am using Quickbooks Desktop 2019.

I have a company who happens to be both a vendor and a customer. They are the vendor when I order my parts, but they are a customer when I turn in an invoice for warranty work. Yes, I have them named slightly different to be able to keep them separate. 

I did a warranty job for them, and ended up using a very expensive part that I had in stock, so I did not need to order the part. I billed them as a customer, but when I received the check I realized that they had cut me a little bit short, as the incorrect part number had gotten entered somewhere. Their reimbursement for the part was less than what I actually paid for it. I contacted the company, and we corrected the error. They said they were going to initiate a chargeback for the difference, but since there was no purchase from my end, the chargeback would be applied as a credit to my account.