Split payments into multiple options

Hi,

In Belgium, a lot of people employees receive ecological cheques from their companies. These cheques have a value of easily 100 euro’s combined, but can only be used for the purchase of ecological products, such as bikes.

I want customers to be able to choose to pay a part of their final shopping cart bill with a self-chose amount in coupons (which have to be mailed by post to the shop) and then the rest with whatever other payment gateway possible.

Does anybody knows a plugin of way to achieve this?

Thanks in advance!
Tom