I'm from Brazil and I'm going to USA in January, so I need to buy dollars.
Here in Brazil every currency exchange done have to pay a tax that is usually 7%.
But we don't have to pay that tax in cryptocurrency. 
So i was wondering if I can do it through Bitcoin or other cryptocurrency.

* First I would buy Bitcoin in Brazilian currency to my wallet through any exchange (Real to Bitcoin)
* Second I shall sell this Bitcoin in any exchange that accept USA Dollars. (Bitcoin to Dollar)

But I don't have anywhere to keep this money. 

Have anyone already done this in any currency? 
Would you describe how you've done it?