To do what you want you need to have both a price and option attribute in your product. In the content object you set the price to 0 and add options for each Full Price and the Partly Payment price.
I tried with option thing. But it didn't work properly. Then I have gone through the php files and realized that if there are two prices it takes the first price in the attribute loop.So I have added some conditions there. I have changed the basket form (Added new hiiden fields), basket.php added some sessions and ezbasket.php.
