It depends on your situation, but simply multiplying any value by -1 will turn it the opposite of what it was before.
You could create a custom function to do this if you have FileMaker Pro Advanced. I'm not sure of any other function that would do that.
0 - MyNumber works also, provided all your MyNumbers are positive.
I'm not sure about "to a number" ("negative value" is also a number), but Abs() function return always 0 or positive.