Quantity Discounts

Hi Friends,

My client wants to introduce quantity discounts based on accruals of 3 months. Suppose if the target quanity procurement exceeds 300 tons in 3 months, then 20% discount will apply, if it is between 200 to 300 tons, then 15% discount will apply. if it is between 100-150 tons, then 10% discount will apply. if it is below 100 tons, then no discounts will apply. So dear friends, how to configure this and what else i have to maintain in info records etc. please explain in detail. full points will be awarded....