Hello issai178,
That's a great question! We found the following information about what happens when you're not able to return the original device "If Apple requires return of the replaced device or part, Apple may require a credit card authorization to serve as security for the retail price of the replacement device or part and applicable shipping costs until you return the replaced device as instructed. If you are not able to provide credit card authorization, service may not be available to you in which case Apple will offer an alternative arrangement for service. If you fail to return the replaced device or part as instructed or return a replaced device or part that is ineligible for service, Apple will charge the credit card for the authorized amount.". These details can be found in section 6.c in the following guide for plans purchases since September 7, 2022 in the US: AppleCare+ for iPhone PDF - Apple Support
For other regions or purchase dates, check here: AppleCare+ for iPhone - Apple Support
Since this is a public discussion forum, we do recommend reaching out to Apple Support to discuss this with them: Contact Apple Support
Thanks and take care!