... its probably a rat, I know, but I want to know if the offer I have received is legally possible.I want to sell a property to a tenant, but he doesn't have much for a down payment.

The place is unsafe to live in, What makes it right for HUD to approve and say it is safe?

I'm curious as to whether that raises some potential legal issues or problems if, for example the lender calls the note due upon learning about the sale? 

After they are approved, I add them to the lease and make them fully responsible just like the original tenant.

So I picked up a 4-plex a couple of years ago, and thanks to a highway getting approved for development in that town the value has jumped roughly 50% in what was an otherwise stagnant market.The property is cash flowing a bit, but nothing special considering some unexpected repairs last year.So my question is: would you think about holding on to it and see where the market settles?

What would legal action look like to recoup some of the costs?

Just have to edit it and run it by legal/PA then I’ll release it.

*This post is not legal advice and is not to be relied upon. 

I am super interested in the notion of sandwich leases, but I am unsure of the legality here in NY state.

But I don't think your landlord is legally obligated to for the reason you stated (TDY orders for training).