Mr. Strauss will mark this debtor in Chapter 3. Head to Rhodes and find Hughes behind the Undertaker building, beside the bank. After talking to him, he’ll lead you back to the nearby church.
Upon reaching the church, you have to drive away the civilians nearby so Hughes can start working on how to get your money. You can ask them nicely to leave or threaten them.
After clearing the yard, talk to Hughes again. He’ll then ask you to guard the church entrance and prevent people from wandering in. After driving away one woman, two knuckleheads will arrive and will ignore your request.
Beat them up then loot their bodies. Return to Hughes and he’ll be done digging up a fresh grave. Talk to him to receive the Diamond Brooch which will be enough to pay off his debt. Collection complete.