Looking to buy a house, how long should I wait?

There is a house for sale down the road from me. The owner of the house is a very good friend and offered to "tote the note" for me. This was 6 months ago, and he also wanted 3,000 down, which I didn't have, so I ped up the offer. Well, a month ago, the house hadn't sold, so I went back to the man and offered him 1,000 down and would put new windows in the house. Problem is, the DAY before I went back and offered him this, a man cane buy and offered to buy it. This was a month ago, the man has not bought it yet and the owner has not accepted my offer. How long should I wait before I just give up and find something else? It's a nice house. 4 bedroom, 2500 sq feet and 2 acres of land. He only wants 30,000 for all of it. But he's also selling HIS house which shares the land with the other house he owns and the other guy is wanting to buy both houses and all 25 acres. What should I do? Just be patient or is there some kind of contract I can make with the seller to prove to him that I'm serious? And is there something I can do about the time line and how long I plan on waiting to see if the other man's offer goes thru?