never coming back to tmobile- Cancellation charges for minor paint spots on booster

I am reaching out to this community with the hope that you can help me regarding the incorrect charges t-mobile has applied on my account (detailed below). I was a loyal customer with t-mobile for more than 5 years and never defaulted on payments.

In April 2024, I have decided to move out from t-mobile and have diligently returned the signal booster without any physical damage. 

Then t-mobile has charged me ~ $300 for the damaged device. When I see the pictures, there are minor paint spots that can be cleaned with damp cloth. Still I am being charged $300.

I believe this is too harsh for a loyal customer like me. I tried to convince the customer service agent multiple times but I am told clearly that this cannot be waived. Also threatened me about collection services.

Any suggestions? Did anyone faced the same issue?