The last time I called my CC company to complain about an overcharge (merchant filled slip out for one price, I signed, but when submitted to VISA it had new numbers on it..), they told me they (that particular CC) NEVER refund a difference. Either the charge is valid, or it isn't. So they refunded the entire $855 to my account.

I never heard from the merchant again, and the woodworking tools in question are still going strong in my shop.

Cheers