Discount based on the chosen payment system

[jacky75]

I'm just requested to implement the following feature: based on the selected payment system, a discount should be applied.
E.g. Payingwith "credit card", if the order amount is greater than 500 Euro, a discount of 5% will be applied. Paying with cheque, if the order amount is greater than 800 Euro, a discount of 2% will be applied.....

Is there anyone who have any idea about implementing this feature? The two issues to solve, in my opinion are the following:
- how to apply a discount at the end of the checkout process?
- how to apply this discount trying to avoid modification in all already existing involved payment modules?

Regards,
Jacky75