Metro 2033

This achievement is called "Realist" with a description of "A coin for a hungry kid? Get a job."

Once you get past the point at the beginning of the game when you're first in the tunnels and get attacked, you'll end up at the station where you're deemed a hero and everyone proposes a toast for you (of course, it's vodka). When you're able to get up, pick up your payment, then just follow your compass downstairs and to the right. You'll come to a kid that will tell you that someone wants to see you, and he'll show you where he is for one bullet. You'll get the choice to give the bullet or refuse. If you refuse, the kid will walk away and the achievement will pop.

THEICEMANCOMETH has a great description but there is a little extra that can help you. As soon as you get this achievement load your last checkpoint and give the kid the bullet this time around. This way you can work on the achievement called "Generous".

If you missed this achievement during your initial play through like I did simply load up CHAPTER 2 "BOURBON" Mission "RIGA" after the cut scene grab the ammo on the table and head down the stairs. Look to your right and approach the kid. While talking to him press (b) and the achievement should pop!