We don't want discount rates change after total value reduces

We have stair price policy.

For the total value between 0-10000, we will give a discount, 10% off.

For the total value over 10000, we will give a discount, 15% off.

Customer bought goods for 10000 dollar and got a 15% off.

We shortly shipped goods, and customer didn't want the rest.

We cannot change the discount to 10% in this case.

How could SAP realize this kind of function?