Roughly around the time you finish the mission "Paper Trail", you can meet a guy called Hossan who is one of Niko's friends from back home. You may have emailed him earlier in the game. You can find him in Suffolk in Algonquin. He'll appear as a person icon on the radar. Go chat to him, and after the cutscene, he'll tell you he's going for a walk to collect some money. He wants you to walk with him, so follow him on foot.
After about five steps he'll tell you that you need a car, so jack one and pick him up. The guy he's going to see is on Castle Drive, just along the road, so make your way there now.
Park in the space, and watch as the guy tries to escape in his Banshee. Wait for Hossan to get in your car then speed after the man and drive-by his car until he bails. Once he does, kill him, then get out and grab the money. Get back in your car and take Hossan to an alley way in Suffolk. It doesn't matter if you have cops on you, just get him there and you'll finish his task.