I know that in sub-to deals, the seller holds the loan in their name while the buyer name goes on the deed and they make the monthly payments and should the buyer default on a payment, the deed returns back to the seller.

I run into this about once a month with clients.This is usually how it goes,I tell the client it will cost $20k for the property if it was updated, but $30k-$35k with those panels.Buyer goes back to the seller and states they need to renegotiate.Seller doesn't believe and says "talk to my insurance company as they are charging less than that"Buyer calls sellers insurance agent and the agent states "yeah, this is a grandfathered policy and we if have to issue a new policy for a new owner, it will be about $35k"Seller then either cancels deal or negotiates down.As @Matt Popilek states, if the seller has multiple options, it will be more challenging to get them to adjust down.Good luck with it.

Normally the way seller concessions work is that it’s just a dollar amount from the seller to the buyer at closing.

You have the deal under contract but need to find a buyer?

@Gary F. it really depends on the location of the property and what buyers are doing in the area. 

According to the realtor, this transaction has been approved by the hard money lender and title company already.I have set a firm price at $100k, which is a good deal for the buyer, but I'd like the quick sale.Here comes the sketchy part: They would like me to accept payment of $125k with $25k being commission for the realtor, which the realtor is then going to use to pay the down payment. 

For the Average buyer this would have been disaster.. we simply built a new construction on it and did OK.. but this full blown rehab can be a real challenge for many .. its one thing to replace kitchen baths paint carpet roof.but when it comes to structural or these really old houses you get from Charleston north into New England.. etc its far more complicated and risky.

How do you qualify your buyers?

Plus, there's a decent chance the buyer of your property would allow a delayed closing in order for you to better "identify" your next purchase. 

Plans are submitted to city, will start on "in-place" type reno next week for existing bath.Goal would be to find a buyer early so that we can either sell as soon as it is complete, or even sell it now - bonus for buyer would be they can customize a bit if desired (e.g. here are your two choices we include in this price range. or pay more to get whatever custom version you want).