Sounds like this was a trade not actually denominated in Bitcoins but in US dollars and just using Bitcoin as a means to pay.
Before buying anything expensive with crypto, I'd like there to be a legal agreement to be written between the two parties, where one of the conditions stipulates: that the price of the item being sold will be denominated in
Bitcoin (or whatever coin the trade is transacted in), and that the buyer cannot be held liable for price fluctuations of said coin relative to other currencies.
Or, if denominating stuff in Bitcoin is undesirable, a separate terms and conditions can have a clause that says the listing price of the item being sold, the price of which is denominated in a different currency than the one used as the medium of payment, is final the moment the purchase is completed, and that future adjustments in the listing price based on price movements of the currency of payment will not adjust the listing price at which the product was paid for.
I'm not a lawyer, so maybe one of them can come up with a better wording.
Or, you can hire ChatGPT for $0/hour, at your own risk:
In consideration of the premises and the mutual covenants herein contained, the parties agree as follows:
Listing Price and Currency Denomination: The listing price of the item being sold, denominated in a currency different from the currency utilized as the medium of payment, shall be deemed final and binding upon the completion of the purchase transaction.
Future Adjustments: Any subsequent fluctuations in the price of the currency of payment shall not impact or modify the listing price at which the product was originally paid for. The listing price shall remain unaltered despite any future price movements of the currency of payment.
Governing Law: This clause shall be governed by and construed in accordance with the laws of [Jurisdiction].
Entire Agreement: This clause constitutes the entire agreement between the parties, superseding all prior negotiations, understandings, and agreements, whether written or oral, relating to the subject matter herein.
IN WITNESS WHEREOF, the parties hereto have executed this agreement as of the date first above written.