Really liking using Bitfinex, great job.

I've got a question about exchange buy orders.

Example :-
1 - Deposit 1 btc into my account and move it to the exchange wallet.
2 - Sell all at market and end up with, for example, USD 900.
3 - Wait for 1 day, then buy all at market when the btc price has gone down to USD 850.

Why can't I spend the entire USD 900 amount that's in my exchange wallet?
Because it gave a warning at step 3 and said not enough margin.
Can I only spend (USD900 - 10% = USD 810)?

Note I'm not doing a margin trade here, just an straight buy at market exchange order.

Sorry if this has been answered before, just a bit confused by this.