PS budget carryforward

Dear all experts,

Please help me on the following issue on project budget carry forward.

For example, there is a project created in 2012.

Its budget is 1000 RMB in 2012. Actual cost is 200 RMB in 2012. And the commitment is 100 RMB in 2012.

At the beginning of 2013, firstly the commitment (100RMB) is carried forward from 2012 to 2013.

Then the remaining budget will be carried forward. The remaining budget is 800 RMB in this case.

But, if I just want to carry forward 500 RMB remaining budget to 2013.

How can I do to achieve it ?

Thank you very much.

Judy