I suggest you consider structuring this as a decentralized exchange, exploiting OpenTransactions. If you're not familiar with the idea, this is a good place to start:

https://soundcloud.com/mindtomatter/miami-2014-chris-odom-on-1?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+TheDailyBitcoinShow+%28Let%27s+Talk+Bitcoin%21%29