You start the sidequest "X marks the spot" when you first go in the Golden Apple inn and you approach a note on the floor. When you go close enough a text comes up saying:

"To Mr X... I buried the money at the foot of the tree in Nightshade cemetary. 20 Pix just like we agreed. All you need to do is dig it up. Next time do not be so impatient about your payment!"

If you head to Nightshade Cemetary and dig near the tree, you find 20 Pix.

Note however that you do not have to find the note to dig up the money, you can do that at any time.