Old owner extended lease without disclosing

[Daniel Antonetti]

I bought a property in Sacramento CA last year.  One of the tenants rented an apartment right after we got into contract for below market.  His rent was $600 for a 2BR 1BA, market would have been at that time about $850 by their marketing package.  We are currently able to rent identical units for $1000.  The lease that was given to me in escrow gave an end date of May 30, 2018.  But when we tried to raise his rent, the tenant said that his lease went until June 1 2019, and gave us his actual lease.   The old manager confirmed that this was the correct lease.  He was given a longer lease for doing "Favors" which I believe was painting and cleaning his unit and maybe a few other things.

But this extended lease was not disclosed during escrow.  I am wondering what my options would be here.  I do not know if this was done by the property manager without the owner's knowledge.  I think that the property manager just did not want to pay for renovating a unit that was going to be sold in a few months.  My damages are between  $4800 and $3000 depending if I use current rents or the stated market rents that they gave.