Sum Of Current = CALCULATE(SUM('Table'[Current])
, FILTER('Table', 'Table'[ParentID] = EARLIER('Table'[ParentID])))
Combined = IF('Table'[Sum Of Current] > 0, 'Table'[Current], 'Table'[Budget])
Combined = IF(CALCULATE(SUM('Table'[Current])
, FILTER('Table', 'Table'[ParentID] = EARLIER('Table'[ParentID])))
> 0, 'Table'[Current], 'Table'[Budget])