As a "tenant" to Fannie Mae, you are given 1st opportunity to purchase the home you are renting BEFORE anyone else does. Ideally, you should call the agent who placed the notice on the door and let them know you are interested in buying the home you are leasing. You are subject to the valuation and assessment Fannie Mae puts on the home, which may or may not be in line with the short sale price, if indeed it was too low for the market condition.
You should absolutely pay your lease to Fannie Mae. I wouldn't necessarily hold your hopes out for the short sale to close, as there is no guarantee what happened before can't happen again.
Best of luck,