There are three ways:
From the "Rules & Rule sets" screen, add the rule "Compare absolute value". This will not change your data, but will only match when they are opposite values. Note that this would match both -10 with -10 and +10 with +10, an outcome that sometimes may not be acceptable.
From the "Rules & Rule sets" screen, add a "multiply" rule to your field and use the value "-1". This will not change your data, but will only match when they are opposite values.
From the "Match fields" screen, use a rule. For example: "If quantity is less than 0 then set to -1 * quantity else set to quantity". Note that this will change the value that the results show. If you want to see the original value in the results, add your field to as a reported field.