Have the filmmaker take you for a cup of coffee so you can learn about the project and get a sense of how responsible he/she is. On one hand, you could be helping out someone on a shoestring budget whose vision would truly benefit from a specific prop. On the other hand, you could be setting yourself up to be disappointed by someone who will be reckless with your bike and cause a lot of damage (and not be able to pay for it). A full-fledged operation would simply offer to buy your bike if it was really what they needed, so this arrangement will be less formal than that. You have to decide how much risk you're willing to take.