Help needed, two separate deed on one property? Current buyer looking to sell me half of their land.
Hi guys, 

I’m interested in one very special deal. 
So here’s the deal,

- 4 acres land in a town shopping center.
- 30,000 sqft commercial retail building on top of it. 
- very small town (population ~15,000)
-asking price $990,000
-it’s on listed for almost 2 years.

So here’s the special part according to listing agent:

-current buyer only wants 2 acres of empty land and looking to divide the other 2 acres land with building to sell separately. (Two title deed?)
-the listing agent told me it will be a simple purchase on my side.
-they are asking for 620000 from me.

So what step should I take next?
Is counter offering 400k too much? Very small town with no comparison sales that I can compare with. 

Thank you for taking time. Sorry for my typo, English is not my first language. 
Any advised is appreciated.
[{"id": "hjhv3j0", "text": "It needs to be properly subdivided with the town or county and that would create another title with an official address, etc, otherwise they\u2019re condominiumizing it and that still needs to be done properly. Talk to county/city planning for the right way to go about it. There will be time and fees associated with this. You need an RPR by a surveyor. It involves confirmation with transportation, drainage, a bunch of services beyond just issuing a deed.", "replies": [], "depth": 1}]