Rebate agreement- Recalculation for previous scales

I have a requirement to recalculate rebates on already accomplished scales.

Te requirement is as follows,

Say a customer has a scale for 0 to 1 million in a period for which he receives 1 %.  Now there is a second scale from 1 Million to 2 Million in the same period for which he receives 2 %.

Now since the customer has acheived both the scales, the company wants to give him 2 % on the entire 2 million instead of giving 1 % on the first million and 2 % on the second million. Some pointers on how this can be acheived would be appreciated.

Thanks,

ES.