I also send an e-mail acknowledgement.

An email or letter from the tenant acknowledging what the rent should have been would work. 

If (after your conversation, as Marcia suggested) the tenants are going to leave anyway in a couple of months,  then leave the rent as-is, month-to-month, or raise it only very slightly.Here's a tip that we learned the hard way, though...if there is construction elsewhere in the building and tenants in a unit *not* being worked on complain about that, have an agreement signed in writing that they acknowledge that their rent is under market rate and this includes any concessions they might ask for down the road for construction-related inconvenience (we had to put on a new roof, which turned out to take longer than expected- and the already under market tenants complained and got us to give them additional rental concessions because we weren't explicit enough).

I received a copy of the leasing agreements to confirm what I was told was documented, and this is what I found: According to the lease the owner has acknowledged the following appliances: refrigerator, stove, microwave, and dishwasher for Unit A.

Here you go:http://www.lonestarlandlaw.com/subject-to-transactions.htmlhttp://www.cleverinvestor.com/education/investing-in-real-estate-creatively/subject-to-sub2/I think we may have been on the same page all along but were getting caught up in the terminology.I acknowledge that an interruption in Sub2 payments exposes the investor to the risk of litigation, and I do not know to what extent good Sub2 documentation constitutes a strong defense.

Tenant acknowledges that Owner or his representative, has explained terms of Illinois Homestead Act with regard to real estate taxes and Tenant understands said taxes are being paid by and through the monthly rental fees alone, and Tenant does not pay said real estate taxes other than through monthly rent."

it was acknowledged that historically it was a legal 2 unit. 

Also I acknowledge I've seen a jump in plumbing costs since 2017 in my area.Knowing only what you're telling us I'd take it out of their deposit and I'm a guy who hardly ever takes anything out of a deposit.In my leases, I'm only responsible for clogs between the unit and the street.

The legal document he will sign is called an estopple agreement and the new owner signs it as well basically saying he acknowledges that there has been a sale and the new owner agrees to uphold his lease. 

You cant work with BROKE.Best to acknowledge it aint going to happen  and implement plan B or C or D if necessary.I was looking a record $120,000 income for me this year.