From what I've gathered from comments: you can go negative. If you do, and you have other subaccounts /staked coins, you have to deposit more money to zero it out in order to withdraw those funds
I'd stay away all together
I can say that in my case i had negative balance and it tuned back to 0 in a few days. So yeah, you can have negative balance (i think only when you used crypto as collateral) but then ftx turnes it into 0. So
Please check ticket #28769, I had a liquidating cascade in 2nd of June for GME, where it liquidated me 30% above index price and in the end I got negative balance.
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