If an outgoing tenant releases possession of the property to you late you'll be unable to give the incoming tenants possession of the property on time.

Seller shall have water, gas, and electricity on for Buyer's Inspections, and through the date possession is made available to Buyer."

After that he can either buy it from me or we will take possession.  

This states that landlords (after giving appropriate notice) may take possession, and dispose of tenant’s personal property.

If the relationship were still good and the property didn't require any work prior to new tenant taking possession, then I probably would.  

Simple: have the seller provide the unit empty upon possession.

The use, possession or sale of illegal drugs at the Dwelling is prohibited.

Focus your efforts on demonic possession, orgiastic Satanic cult rituals, black magic, murder-suicide storylines.

You could buy the property, take possession, and then resell to him.

In this case this is multi million dollar company with tons of oversight , but you still are not comfortable paying for something you don't have in your possession )  I am sure you know what happens next .