Data migration project issue

Hello Experts,

We have data migration project from legacy system to PS and the user have a
need of maintaining the historical data for reporting purposes. In our project
we are going to use WBS element for planning and budget data.

Here is the issue, project had a net value of 50,000$ and it was already
billed for the amount of 20,000$ in the legacy system.

Please suggest - how should we handle these billed 20,000$ in the SAP for
the users to have full information about the project costs? What CO object
should we use for storing this data?

Thank you,

TJ