Sass negative variable value?

Try it like this

margin: 0 (-$pad) 20px (-$pad);

A more sane solution according to sass guidelines would be to interpolate variables like the following example:

margin: 0 -#{$pad} 20px -#{$pad};

An example: https://www.sassmeister.com/gist/c9c0208ada0eb1fdd63ae47830917293

I'm adding my two-penneth after considering the two previous answers, but then reading this (emphasis mine):

You should especially avoid using interpolation like #{$number}px. This doesn’t actually create a number! It creates an unquoted string that looks like a number, but won’t work with any number operations or functions. Try to make your math unit-clean so that $number already has the unit px, or write $number * 1px.

Source

Therefore I believe the correct way would be as follows, which preserves the mathematical abilities of SASS/SCSS:

$pad: 15px;
padding: 0 $pad $pad;
margin: 0 $pad*-1 20px $pad*-1;