It sounds as if they are doing this under Return Protection, but waiving the $300 cap and allowing the cardholder to claim the price difference in lieu of making the return...

Return Protection Offers you guaranteed product satisfaction on designated items purchased entirely with an eligible American Express® Card. If you try to return a designated item within 90 days from the date of purchase and the merchant won't take it back, American Express will refund the full purchase price, up to $300 per item, excluding shipping and handling, and up to a maximum of $1,000 annually per Card Account. Call 1- 800-297-8019 within 90 days of the purchase date to notify us of your request.