Sounds like you have done a good job of making it marketable to me. If you cant agree on a price to purchase it from him to keep it for yourself then I see your difficulties. If it has been a partnership deal at least you didnt have to put all of your money up along the way so the buy out price could still be a little higher and the deal make sense. You know the property, rental history, cap, and the next deal you may not so sounds a little safer to pay more for the buy out but depends on the #'s I guess. It may not be a price you would buy at normally but your probably use to getting really good deals but if its still a good investment for someone else why not pick it up.
On the other hand it sounds like your offering it in an attractive way using some of the ideas you have offered. Another idea I have used is to offer to sell the property with a home warranty, insurance on the appliances and mechanics, paid for 1 year. It wouldn't be that costly and your buyer would feel a lot more comfortable knowing the mechanics etc are insured. Home Warranty of America is one place that offers these type of warranties. It covers anything from appliances, furnaces, garage doors, plumbing, electrical, door bells, and much more depending on the plan you choose. You just enter the buyers information in on the website (purchase it for them) when you sell the property to them and the plan will be for them as the new owners.
I hope you find this useful in your business :)