You should not have given him your tax ID. There is no reason he needs it unless you are under a W2 contract. Claiming they withhold your taxes for you is more likely a 20% discount for them (they most likely pocket it). Don't expect to get a W2 form. Do not give him anything till the payment clears, even if that takes a few days. If they ask why, just say they have worked hard to earn your distrust.

Here is why I would not trust them:

The Frankfurt Group has billings of over 20mm, I think that's considered a "real" company.

Twenty millimeters? Besides the misspelling, a real company would NEVER use that as "proof". They say they are a registered business, ect.

I'm going by the rules here, brother. I'm trying to make this right.

No business would ever call a client "brother".

My advice is leave now. They aren't worthwhile to work with and are probably going to try to scam you again (they already got your tax number)