will be a glitch. I often get the case where buyer makes and offer, I supply counter offer. Buyer then rather clicking to accept offer hits the original BIN and then complains that invoice shows original BIN price.

Seller cannot change sell price on invoice. 

Dont know why you had to acknowledge. If you sent offer and seller accepts then item is sold, There is no confirmation required from you, it is sold as soon as seller accepts. Also once a seller accepts offer they cant then decline it. Are you sure seller didn't decline and then somehow you selected BIN button, hence buying at full price?