Entering A Negative Expense

I have a small auto repair business. I use quickbooks online. Generally when i purchase parts for a customers car, I do it with my business bank card and enter it in quickbooks as a billable expense to the customer.

Today, I purchased a part (part a for future reference) for a customers car which ended up being wrong. I took the part back to get a refund and purchase the correct part (we will refer to the correct part as part b) part a was 56.30 and part b was 49.84 so they applied a credit of 56.30 to my receipt for part b which left me with a credit of 6.46. How do I enter this into quickbooks in this case?