This is a limitation in PPM. There is already an enhancement request for this functionality.
ITG Product Enhancement - #185637
However, enhancments are not normally provided as part of a patch update, and they are reviewed as part of the planning for future ITG releases. As such, I do not expect there to be a quick turnaround on an enhancement request.
I would recommend you add another budget line into the budget table and add the remaining value. For example, if you needed to add a budget line of 100,000 ('000) then you could enter this as 2 line of 50,000 ('000) each.