Purchase a renter occupied auction with an unfair existing lease

[Helen Zhang]

I would like to learn an edge case study of "what-ifs" that I've always been wondering about as I think this can be easily abused as I am about to purchase an auction house that is occupied. This might be an interesting case (not what I am encountering) but I would like to know all the legality around this abuse. 

Assume that the owner of the property is currently about to be foreclosed, he then wants to minimize his lost by renting the property out and take the tenant's deposit and possibly collect a few more months of rent before the property is being foreclosed. He then signs an unfair agreement of $5 dollars per month lease for 100 years with the tenant. 

As the property forecloses, and you purchased the property. The lease would survive and must be honored. Is this correct?  I wonder what would be the legality around this issue and what needs to be done here. 

This seems to be a pretty bad abuse. Of course I am exaggerating that the monthly rent is $5 which is obviously not a fair market price, but what if it is on the very very low end of the market price and for a very long term that is 5 years? who is there to judge whether this is legal?