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.
Minor: You can't spend the entire $900 on coins because you also need to have enough funds to cover the transaction fees.
Major: You must specify the quantity of coins that you are buying. You can't put a "give me as many coins as I can get for $900" order; you have to specify that you want 1.04 coins.