Row level calculation over the dimention

Without having your source dataset to work with, I can only attempt a theoretical solution... have a try and see if this works:

Create a measure called "DIFF (PLAN-SUMM)" :

=VAR isInRow = HASONEVALUE( tab[Type] )

RETURN SUMX(

       VALUES( tab[Type] )

       ,  CALCULATE(

              VAR thePlan = SUM(tab[PLAN])

              VAR theSumm = SUM(tab[SUMM])

              VAR DIFF = thePlan - theSumm

              RETURN

                      IF ( DIFF >= 0, DIFF

                         , IF( isInRow, DIFF

                               , 0  )

                          )

              )

       )

This measure should work in both the grand total cell and in each row.

When it's in "row mode", all kinds of DIFF's will be displayed, but

when it's in grand total mode, only DIFF>=0 will be included.