Once the company completes the feature satisfactorily they get the cash. Or perhaps even a situation where they can grab the cash to start the feature- but this is obviously higher risk- like kickstarter- but the risk could be managed and even help the feature development process.
PS- With this in mind, importing feature requests from other sites could help growth. IE: People see a GREAT feature THEY want and are willing to put in $5... as compared to coming to the site to submit their ideas. Another idea: Use ad revenue from site to 'seed' a user's initial ability to donate $$$. Perhaps require CC input in order to contribute this 'free money', thereby making it easier to give real money. I know the free money would have to be incredibly small, but sometimes little acts of giving something for free can really change a system. Just some thoughts...