Can't be done....unless I'm missing something (still on 1st coffee!)

Sum of the 9 digits = 45;

a + b = 45

a - b = 10

========

2a = 55

a = 27.5

Are we allowed to use the likes of 1+23?

1 to 8 works; sum = 36:

a + b = 36

a - b = 10

========

2a = 46

a = 23

So 23 and -13:

1 + 2 + 3 + 4 + 5 - 6 - 7 + 8 = 10

1 + 2 + 3 + 4 - 5 + 6 + 7 - 8 = 10 ....and a few others...