Wiki says that the system expression "sign(x)" returns x/abs(x) for non-zero numbers. That's -1 for negative numbers, +1 for positives. This suggests the following (untested) expression for "move from a given value towards zero by a specified amount":
Note that this will "overshoot" zero if abs(Value) < Amount; e.g. with Value=1, Amount=2 you get -1.