Running into problems doing due diligence , please advise!

[Grace Wang]

Hi BP community, 

I am currently doing  my due diligence in acquiring a 4-plex, and running into issues:

 How do I verify rent if tenants are paying cash? 
How do I verify lease agreement if the seller lost the lease agreements?

 The bank is not willing to lend money until they see a lease agreement between the seller and tenants.

Would it be a problem that if there were no exiting lease agreements prior to me acquiring the property? Such as unable to evict a tenant? 

Please advise.