I believe a Flex- Option is what you are looking for. You would have to negotiate with the seller a price and deposit. Usually a small deposit will secure your interest in the property so the buyer can't circumvent you. You just tell them that you have it tied up under contract and you will assign the contract to them for a fee.
This agreement also let's the seller to continue to market the property and gives you the right of first refusal, which basically means they have to contact you first before they sell it. If you don't have a buyer they can sell it to theirs and you would lose your deposit.
This is great if you are worried about getting a buyer and tying up the property, it gives the seller the best of both worlds.
Hope this helps......