Can I legally issue a refund to my customer when the funds came from their lending company?

We recently completed a project where we supplied a new retail business with equipment and furniture. The original order was close to $100k. The customer received an equipment loan from a third party lender for the items they purchased from us. However, after we sent the lender invoices for the goods and the lender paid us in full for those invoices, the customer later decided to cancel some of the items in the original order (nearly $20k worth of goods). Now the customer wants us to issue them a check back for the cancelled items. Is this legal to do? And if so, could we do it from a tax standpoint where I'm sure the lender will 1099 us for the full original order amount at the end of the year. Thank you for your time - your answers are very much appreciated!