Rebate Accurals Issue

Hi SD experts,

I have created Rebate Agreement with to Scales and Scales qty as shown below:

to-- 10.00 L      1.50 USD

       20.00 L      2.50 USD

I have created order and invoice with 11L Quantity in invoice for Reabte condition system calculates it as 11*1.50=16.50 USD. but as per record it should calculate as 11L*2.50= 27.50 USD.

when i check Accurals field in agreement it shows as-1.50 USD. if i make it as-0 then it calculates on value-0. in settelement it shows different value.

can some one explain me if i am doing wrong and explain me how i maitanied accurals in rebate condition records and also explain me rebate calculation procedure.

Regards

BJ