How?Unauthorised modification

My device is not charging anymore and is dead now.

I was charging it with the Apple's Charging adaptor and cable, suddenly the battery % dropped from 49% to 1% and phone still showed charging symbol but it wasn't charging and then it got dead.

I recently bought the device so it was under warranty, then I took it to the Apple store for repair, they sent the device for repair and the repair centre said that there's unauthorised modification made in the device,

so I was given a quotation 10,400HKD to repair it or I'll get the device unrepaired.

Fact: I bought it for 11,900HKD.

I never gave my phone to any other service center,

The phone was never opened as there's no point of doing it as it'll void my warranty status.

Aslo, there's no physical damage.

How can there be a modification if the device is not opened by anyone, other than Apple itself?

When I asked for what modification was made they didn't have any specific reply at all.

I asked for further investigation but the result was same, unauthorised modification.

Also Apple void my warranty status.

What should I do now?