When you are low on money, go by Roman's cab depot, and an F will come up on your radar.

Go to it and a guy will be leaning with his back against the wall.

Go up to him, and him and Niko will have a chat. He will then give you $100.

Keep doing this until he's not there any more. He is the guy from the intro who is talking to you.

NOTE: You must die before he will be there again.