This quest can be acquired from Kunesh, who is begging in the streets of Rattay.
Note: You will need to have a spade in your inventory to dig up the treasure.
Enter Skalitz from the north entrance and go to the position marked on the map below.
Dig the ground under the dove-cote and loot the treasure within. You will find Beran's cup and 50 Groschen. The quest will complete at this point, but you can now go back to Rattay and give the cup and Groschen to Beran.
Note: You can also give the cup and Groschen to Beran's wife, but she will not give you a reward for them.
Having given the items to Beran, he will thank you with 20 Groschen.