Battery changed from apple I care, now phone is total dead.

I have given my I phone 7 mobile on 06-06-2022 in imagine apple service center with working condition , they have told me to check whether the stock available or not and they have checked and told me stock is available we can change the battery and got it 4000 rs from me to change the battery and they've asked me to come and collect the device after 2hours. when i came again to the store they have told me to your devices has hardware issue we need time to resolve the issue I've accepted and given time for two weeks , but now they are saying your devices is rebooting continually because of used unauthorized modification, we cant do anything pls collect your device and go will refund your amount which i given amount for change the battery, before getting my device they have to mention or conform with me but they dint say anything the conditions while i submitting the device, if they would say before I submitting my device i wont be given, but they are saying now , how can I accept like this kind of issue which they have made. if apple cant solve the issue then who can solve.

Kindly suggest the solution or shall i throw my iPhone into trash box????????.......

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