We inspected it on site this morning.Average rent is $550 like the lower one is renting for.

However, we were informed today that our closing has to be pushed off to a far-off future date (rate will no longer be locked and basically will have to do the financing process again) due to an issue we haven't run into before, but seems simply not legal to us, so we hope we can get some assistance.We only know as much as we have been told, so here we go... the home we are under contract for was sold to the current owners via an owner carry, so the note is held by a private individual (the owner before the current owners).

Would he also have to be present during the closing date?

In the sale of a REO in the mls, not on an auction site, yes owner occupants have an exclusive period in many/most cases.

There's a good prospective tenant that wants to sign our lease but they are not sure about start date (work travel schedule) until 3 weeks from now.

I gave you my examples with real numbers and dates. 

There are many online sites where you can pay$65ish and they will send you a fabricated letter.

Welcome to the site Adam.

Hello and welcome to this site Vinog!   

Keep in mind, in a typical sale it is assumed that seller will pay all taxes up until the date of closing.