I would lke to know how you are gathering the input. My thought was you should not need the ABS function.
If you had options buttons -5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5 then it would be pretty straight forward and you would not need this validation, you could just use the values of the option buttons.
I am assuming then they select positive or negative and then enter a value or just select a numeric option.
You would put logic in a rule that said if selection equals negative 0 - numeric ( which would give you your negative value) otherwise the numeric.
Let me know what the data values you have or attach your form and I can maybe help you better.