This is done in the Marakesh mission. You will need to give a beggar near the mission start point 5 coins. To begin, you can take three coins as part of your equipment; however, if you choose not to, you can find more coins in the mission. To the left of the start and past the crowded market, you can find a shoe shop with soldiers outside it. Sneak inside and on the counter you will find 5 coins. Head back to the beggar and throw the coins onto his plate. This will unlock a challenge and your achievement / trophy.
Here is a video guide with English commentary for this achievement!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUK9Qh-6TvL3...