Volume discount for whole month

Hello sir's

I am having one querry,

i maintain the time dependent condition Volume discount (zvls),

For EX:

I want to calculate the zvls for aspecific period,

suppose i created 10 nos of purchase order, post a GR and also settled the payment for that vendor in 1 month period,

now my purchased quantity is reaced 500 Kg and i set a discount quantity based

Qty Disc

100 20 Rs

200 22 Rs

500 25 Rs

Now i want to take the discount from vendor,

what i suggest that its only possible only in credit memo against that vendor with all other values taken manually.

But they want to pick the condition for whole month automatically and calculate the discount for quantity for that period.

plz suggest.