Purchase Order Price based on accumulated used qty from contract

Dear Gurus,

We have a requirement from user where we want to get the price based on contract qty consumed in PO.

For example:

In purchase contract ((type WK)) line item pricing scale as follows:

qty 0 - 100 : price 500 USD

qty 101 - 200 : price 450 USD

qty 201 - 300 : price 550 USD

Currently we have already past PO with this contract line item:

PO 1 : 50

PO 2 : 75

By this time my qty used from this contract is 125 pc.

When we create new PO with the qty of 50, I get the price of 500 USD instead of 450 USD.

How to make the newly PO to take the price scale 101-200 (450 USD) in this case ? What configuration is required ?

Appreciate your help.

Regards

Rony