The Yakuza run a protection scheme for certain businesses, and it's time for them to pay. You need to pick up the cash.
Make Them Pay
Grab a car and head to Torrington to pick up the first briefcase. Next head to Bedford Point and pick up the next one. You may want to dispose of the ridiculous gang member trying to shoot at you as you pick it up. Next head to a restaurant near Belleville Park. He will tell you that a gang robbed him and have his money. This gang is the Diablos, so you need to drive all the way into Portland and into Hepburn Heights to take it back. Drive across the Callahan Bridge then into Hepburn Heights. Kill all of the Diablo's guarding the briefcase. A grenade works wonders as does a drive by. Once they're dead, pick up the briefcase and head back to the Casino on Staunton Island.
Mission Passed!
$10,000
Additional Notes
You don't have to kill the Diablos, you can just run in and steal the briefcase. Of course it's pretty much a law that you kill everyone in Grand Theft Auto, so you might as well do it anyway.