Not an 'official' one like they do in large commercial negotiations, but a letter that is kind of personable and states some things you like about the house/property and a few ways you would be interested in purchasing it.  

I found a fsbo property for my buddy and he agreed to give me a 10% stake in the property if I negotiated a good price for him.

My experience is to do whatever negotiation in court and have their response recorded ( if they are leaving by when they will give you possession etc)

That requires more aggressive negotiation because it's buying and selling only.

See if you can negotiate a price that works for you and get it under contract with the understanding you're using conventional financing.

And you can also negotiate with the seller for credit. 

It is like any other negotiations, if you have the only roof top in town, you have the power.  

But I'm sure he'd negotiate if you offered to purchase more than one.  

Is there any room for negotiation during a sale like this?

They are legally commited to buying three years out at a negotiated price.