After some of the early missions for Roman, you'll notice a small icon appear on your map near his taxi rank. If you head to the icon, which is shaped like a person, you'll see a small cutscene where you'll meet a random pedestrian. During the cutscene he tells you he's rich, and eventually gives you a random amount of cash between $100 and $500.
Later on in the game you'll find the same guy a block or two away from Roman's taxi rank. Walking towards him this time will allow you to do a mini-mission.
Grab a vehicle and pick up Brian. He now wants you to take him to Masterson Street. Follow your GPS to the destination. Brian will get out and make a drug deal with some guys in an alley, then once he's got the product, he will get back in your car.
Now he wants to be taken to his home in Wappinger. Follow your GPS again and make your way to the destination. Once you arrive, he'll say thanks, give you $200 and get out of the car.
You'll eventually meet Brian later in the game when he has dropped his drug habit. He wants you to take him to see his old dealer and pay off a debt he owes him. He wants you to go with him, so grab a car and drive him to see his dealer. He will speak to his dealer, but they'll turn on him, so run over the dealer and Brian will ask you to take him home.
He'll eventually give you $500 for your trouble.