HELP!!! Need some serious advice!

[Zipp Huntere]

I am currently working on a project in which the seller has agreed to sell me his property at a substantial discount, the only thing is that he is in the process of moving his stuff out the property and needs some assistance. Initially, it was a Uhaul ($250-300) rental, but now he is looking into acquiring a trailer ($5K) due to the volume of stuff that he has (hoarder). He is asking me to front the bill, which I have no problems with as it is with the understanding that any assistance will come off of the purchase price. Ideally, I wouldn't mind putting in $250-500 with the understanding that it is to be repaid if and for whatever reason we do not close. My question is how can I structure the remaining $4500 so that it is not paid out of my pocket and is paid at closing?

Any advice is sincerely appreciated.