That is all correct, but he could have removed $50,000 before. That is his money. The shorting of stock creates an extra credit balance that is not his money. I wanted him to know, which I think is what he was asking, that he can't borrow against the cash from the short stock.
Shorting stocks on margin, and then withdrawing any excess funds.... ---> recipe for disaster. The stock spikes up out of the gate and then continues up on low volume.... your broker covers for you. You lose your ass. Then it drops like a rock, because you were right. Search VZ---- IRBT at $130. Or Sinclair Media at $63.
No when you are in a short position, you are already trading on margin so your cash is pledged against it and therefore not available for anything like withdrawal or margin loans. You will see when you are on the cash withdraw screen, any cash that's equivalent to your short positions won't even show up in your available balance for you to withdraw. You can however sell some of your stocks for $15K and use that money to buy the car but you can't touch any money that's pledged towards your short positions.
I think there may be other positions that are blended into the OP's account. When I shorted with IB the cash proceeds was reserved and unavailable while the short was on. The profits for the position were netted against other profits so I could actually withdraw more overall, if that makes sense...
I advice you do not because the funds in the account resulting from the short sale serve as collateral for the delivery of shares sold, even though you do not currently own those shares. Also you cannot withdraw the cash you get when you sell stock short.