What happens when an appraisal is higher or lower than the purchase price?

[CJ Moulton]

Hey BP,

What happens when an appraisal is higher or lower than the purchase price?

Lets say you put in an offer for a house at $200k, and it gets accepted. Once the appraisal is complete, the value is now $150k. 

What exactly happens to that $50k? and vice versa, what if the value is goes up to $225k? Where does that $25k go?

Any advice is greatly appreciated! Much love.