Net value for contract item is wrong when billing is done for one the period

Hello all,

We are facing an issue with contracts with periodic billing.

Through VF04, when we are doing the billing for item 10 and 1^st period successfully, now when we come to VA43 to see the contract details. For the Item 10 the Net value for the billing is not considering the Billed period.

Ex: period 1 value is 100 and same value (100) till period 10.

Now since we have done the billing for period 1 , the net value is shown has 900 instead of 1000.

Please suggest how to fix, any BAPI or program where this calculation is done .