The common problem with buying second-hand.

The problem is, recently I was deceived like a child with candy. Essentially, one scammer deceitfully made me set up his iCloud to lock his lost iPhone, but in the end, he locked mine and demands $100 to unlock it. I have both the iPhone 12 Pro Max itself and its original box. I asked a friend to help me with this situation, to which he replied that no one will help since it's an iPhone without a receipt... What should I do? I have a brick from Apple that I can't use.