turnover discount

How to do this type of discounts in navision 4.0 SP 3.

the example is suppose A is Vendor.That he announced to customers If customer purchase Item B this much amount with in a year customer will get this much discount.

Annaul Purchases 0 - 10000 INR discount - 10 %

Annual Purchase 100001 - 150000 INR discount - 30 %

So at the time of purchases customer will not get any discounts. Finally he need to check the total year , how much he purchased from that vendor for that item.So finally customer will issue a debit note to that vendor to get that discount if it crosses the amounts > 100001.

So what is procedure to do this type of discounts in navision ?

thanks and regards

This is a form of rebate and NAV does not handle this as standard. If you search for rebate in the forum however you will find postings as this has been discussed as a modification previously either here or on www.mibuso.com