It doesn't matter what Bob does after he's given Chad the script. Bob gives him the money under that one condition, that is finding a number that returns TRUE once OP_HASH160 <h> OP_EQUALVERIFY is executed. It is ncommonly said that the money go backwards (Dina -> Chan -> Bob -> Alice), but the money actually go forwards (or precisely, the ownership of the money). What does go backwards is the satisfaction of each node's condition.