I am developing a game and I want to use some assets that are available online for a fixed price, I want to sell my game and actually make money off of it. I have contacted the owners and they will sell it to me for a fixed price to use commercially. However, I am on a very limited budget and don't really want to pay up front for the assets.
What I want to know is how to defer the costs contingent on whether I make any money. In other words, they give me the models, and then I promise to pay them back out of the first revenue. If the project dies, I'm not out any money.
Other than my word as an honest guy, how could I convince them to agree to this? Would they charge me more because of risk involved?