Contract and PO Price Calculate by Rebate

Dear Experts,

There is a requirement from customer to calculate price like this,

A contract signed by purchasing and vendor by quantity scale and valid date.

material quantity price date in March

A 500 to 600 price =10

A 600 to 1000 price=8

A >1000 price=6

POs in march.

A 550 price=10 date: 20-3-2018

A 650 price= 8 date: 21-3-2018

total quantity in march=550+650=1200>1000 so price vendor charge after all is 6.

I know there is a rebate function in SAP but price can not get reduce by a certain percentage only certain price for each scale. please give some advice if you can