Fictional currency transaction system

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Project Budget (USD)
$30 - $250

Project Description:
I'm looking for someone to create a simple transaction provider with a working demo (without use of real currency) . Basically:
Site A is the 'provider'. A user can register as a merchant or a buyer. For demonstration purposes the buyer gets $100.
Site B is a demo merchant site. Site B offers a virtual product for $10. The site contains a 'Buy Now' button. Upon clicking the Buy Now button the user is redirected to Site A with either a log-in form or, if the user is already logged in, a request to pay $10. This page shows the total amount of $ the user has ($100), how much they have to pay ($10), and how much they'll have left if they pay ($90). There's also a 'Pay Now' button.
When the user paid the amount for the demo product, it is being redirected to Site B along with a secure callback and is presented with the product (an image for example). This would have to be a secure system.
It's a simple project due to the amount of open source code available that enables the creation of such a demonstrational website with relative ease and short time. In order to develop this I suggest using open-source code like OAuth2 (preferred), OpenTransact or PaySwarm.

Please send me a PMB with how you wish to develop this project (through the use of existing code or from scratch) and what code you'd like to base this on.

Skills required:
jQuery / Prototype, PHP, Ruby on Rails
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