First the warranty me for free, now they are asking me to pay?

So awhile back when my tire grew a budgle on the sidewall, i got it warranty by ford to replace it for free. After 4 months, i get a mail today saying the warranty was denied by Michelin, and that i need to pay for the tire.
should i just ignore them? I don't think they can do much since i already have a receipt showing they did it for free back then. Its like them telling me to pay for their mistakes. They even had the car for a whole week and said a tire inspecter came and look and said its good to be replaced for free.

Who sent the letter?

If you want legal advice, go talk to a lawyer.

Kept the car for a week… A week? Really. Did you say a week?

The warranty is for failures due to manufacturing defects.

Michelin *very* rarely suffer those which is why they can afford to offer a warranty. Every tyre replaced under warranty gets a detailed inspection by Michelin to identify any manufacturing or materials defects so that they can prevent a recurrence.

They have now upon closer inspection found the damage was due to driver error: i.e. You kerbed the tyre. That's not covered by the warranty. If you don't pay now, you'll pay more later when they sue you for the current claim plus any costs they incur to recover the debt.

Your "proof" amounts to zilch: you will have signed a declaration in person or electronically that the damage was not due to an accident or misuse, and that you'd agree to pay up if they discovered the declaration was incorrect.

Michelin really do know what they're talking about and will not have rejected your claim unless they were absolutely certain of their facts.

It's up to you to prove them wrong.