absolute numbers subtractor
Here's the schematic I'd been discussing:







simulate this circuit &ndash; Schematic created using CircuitLab

It was just a matter of inverting the inputs, based upon the comparison output.

It's a little excessive to do it that way, but also pretty obvious when you think about it. (I gather those were the parts you are given to work with.)

A simplification isn't too difficult:







simulate this circuit

But it is a little bit less obvious, until you look at the algebra (or table.)