the $850M number you're talking about is the amount that Bitfenix gave Tether through Crypto Capital to cover for losses and/or missing money. the $18.5M was the fine they paid the NYAG for covering up that transaction.
That's the amount that Bitfinex took from Tether to remain solvent after the $850M was seized from Crypto Capital Corp in the money laundering sting. Bitfinex wrote them an IOU for it. But of course that money didn't exist anymore, it was taken by various world governments, and BFX and Tether have the same owners.
Bitfinex and Tether are the same company, roughly speaking, and I do believe they co-mingled their funds at times.
This is covered in section III of the NYAG settlement [3].
