columns B and C are the trial balance numbers (debits and credits)(sometimes
blank) columns D and E are adjusting entries (always positive)(debits,
credits)
columns F and G should return correct value and should hold the formula's.
I have been trying with :
=IF(ISBLANK(B9:E9),"",IF(AND(B9=C9,D9=E9),B9+D9, IF(AND(B9C9,E9D9),B9-E9,IF(AND(C9B9,D9C9),D9-C9,""))))

but maybe it could be shorter?