should exchange orders keep the original discount?

A customer placed a bulk order (9 items) and received a 30% discount as part of a “buy more, save more” deal. After receiving the order, they decided to exchange 3 of the items for a different (slightly more expensive) version.

I processed the exchange as a new order (pretty standard on our end), and since it was now only 3 items, it qualified for a 10% discount instead of the original 30%. We send the invoice to the customer for the difference accordingly.

The customer is now pushing back, saying the exchanged items should retain the original 30% discount from the first order, and has calculated the price difference based on that.

How you all handle this? Do you honor the original discount on exchanged items, or do you treat it as a new order with current promos?