FI Document Created During Migo

Hello All,

For My Depo Plant,

We are creating PO for Say some 200 qty.

In migo we are changing in exbas value and bed is inputted manually.

and save Document.

But in my fi Document  i  need to display old exbas with 2 % cst added on it.

Please suggest where i  can write the logc ad which are the table for it.

Regards