This quest can be acquired from the miller west of Ledetchko. He has a captive Cuman and wants to know if he has any treasure hidden nearby. To get it out of him you will first need to find a translator. Head north from the mill and speak with the vagrant who is working with Miller Simon.
If you fail to convince the vagrant to help you for free, you can pay him 40 Groschen for the job. Go with him or meet him back at the mill where the Cuman is being kept. Once there, speak with him to begin the interrogation.
Once the Cuman has agreed to show you where the treasure is, speak with the miller. You can then establish that Henry will get half of anything that is found. Speak with the Cuman to fast travel to the location of the stash.
Assuming you chose to have him guide you to the stash, another fast travel will initiate taking you to the location. Follow the Cuman down the slopes until you come to a tree trunk. Open the stash to find 200 Groschen inside. You must then decide what to do with the Cuman; take him to Rattay, let him go or execute him.
Take the stash back to the miller and give him half of the Groschen. Alternatively, you can choose to lie to him if you have enough charisma, and keep all of the treasure for yourself. Either way, this completes the quest.