Consulting a lawyer over $99 is not worth it, unless it's for the principle of the thing.
The simplest is to tell them "It's a mistake", and then ignore them utterly.
The simplest is to tell them "It's a mistake", and then ignore them utterly.
There's the other choice: continue to not pay it. Your obligation is to Microsoft, not the collections agency. Microsoft mistakenly sold your account to the collections agency; that is Microsoft's problem, not yours.
Consulting a lawyer would also be helpful.