Not completely sure what your case is, but you could add an onchange microflow to the enum that the user selects and then add a decision in the microflow like $object/enumattribute = positive if true change the decimal like this
if $object/decimalattribute < 0 then $object/decimalattribute *-1 else $object/decimalattribute
when the result is false from the decision changethe value as:
if $object/decimalattribute > 0 then $object/decimalattribute *-1 else $object/decimalattribute
Maybe this helps in your case.