Ok, do you want to get into debt to the mob, well here you go. First I suggest that you don’t have a billion dollars! Go to any casino and get the highest gambling skill you can get (you'll know if you got the highest gambling skill if you can gamble 1 mill a pop). Then after you get that high gambling rank go to Caligulas (the casino you did missions at, you know, the cosine with a horseshoe driveway), when you get in their loose all your money (the easiest way is to play blackjack and bust all the time) if you seem to be getting very lucky, go out and roam around for a minute and go back in a gamble, you should have worse luck. Then you keep gambling until you get in the red money zone up to a million dollars. Then when you leave you will get a phone call, he will give you a friendly reminder that you owe money. then a short wile after he will tell you that he's sending "collection specialists". Don’t stand around to long or you’ll get a "shooting" surprise. You can only get the surprise in Las Venturas. This surprise happens all the time until you pay off your debt.