Already tried to do that prior july 6... and got:
We can’t cancel your subscription
You have already paid for your Xbox Game Pass Ultimate subscription. It will expire automatically on 11/11/2023.
Had the same problem, keep getting cancellations after a few days, usually is because the sale ended before the seller gets the key.
Also the majority of sellers go thru blue dollars (black market) or dollar turista (tourist) because the exchange rates are higher around 460 pesos per dollar...