Pablo, probably your best chance is to go to the bitcoin OTC market and make your offer. See http://bitcoin-otc.com/

A list of current SELL BTC orders is here: http://bitcoin-otc.com/vieworderbook.php?sortby=id&sortorder=DESC&type=SELL&nick=&thing=BTC&otherthing=&eitherthing=¬es=. You can list your offer and see if someone is interested.

Good luck.