To Tell or Not to Tell - S1-E8
Continuity mistake: In this episode Sally asks Rob if he has a brother, Rob says "Yes, but he is married." However his brother Stacy is trying to become engaged later in the series, in episodes "Stacy Petrie Part 1 and 2."
Suggested correction: No, the Petries buy the house with the rock, but swap it with their best friends and buy the house next door. Therefore Jerry and Millie have the rock in the basement.
Rob has to pay half the cost for Jerry to seal his basement every year. The reason why is because Rob's rock redirects water that ends up flooding Jerry's basement.
The Petries did buy the house with the rock. At the end of the episode Rob explains how their rock caused flooding in the Helper's basement.