This becomes available in Chapter 4 if you’ve already completed the previous missions (which you can also do in Chapter 4 if you missed them in Chapter 3). You’ll find Leopold near an outhouse in Shady Belle, head over and speak to him to learn the name and location of the next debtor, Algie Davison. Taking a look at the map, you’ll see he’s not too far from Shady Belle so head out the back and follow the path along until you reach the marker, you’ll find Algie down at the river there.
Follow him into the house and he’ll begin looking under the sink to find his stash. As you go to claim the stash however, Algie takes a swipe at you so put the poor man out of his misery and then head into his son’s bedroom to find the chest with the money in it. Be sure to loot the other items here, of special note the pack of Premium Cigarettes. Be careful on your way back to Shady Belle, a group of Lemoyne Raiders ambush you on the path. Enter Dead Eye and take care of the Raiders on the wagon before turning your attention to the others. Once they’re dead, follow the path back to Shady Belle and hand in the debt money to complete the mission.