Irritatingly, I keep coming across exports from accounting systems where the figures are always stated as absolute amounts with a separate column showing Dr or Cr to indicate the signage, Dr being + and Cr being -.
It isn't always convenient to add a helper column but a formula in the form of
=SUMIF($A$1:$A$10,"Dr",$B$1:$B$10) -SUMIF($A$1:$A$10,"Cr",$B$1:$B$10) will subtract the sum of all the Cr values in column B from the sum of the Dr values in column B based on the Dr/Cr designations in column A.
I've noted that the range for SUMIF isn't necessarily a single column array. So, if you had Dr/Cr designators in columns A and C and amounts in columns B and D the formula could have the form =SUMIF($A$1:$C$10,"Dr",$B$1:$D$10) -SUMIF($A$1:$C$10,"Cr",$B$1:$D$10).