It's a workaround.... You can obtain the object's scale on each axis, without using variables, by using the following technique:
(ScaleX is an object variable for its scale on the x axis...)
Object.ScaleX = Object.Width / Object.ImageWidth
There are similar object values to use for Y scale (ImageHeight etc).
So, if you want to set the Object.ScaleX to Object.ScaleX - 0.1 (or whatever you want = YourDeltaScaleValueHere) you could do the following:
Object.Width = clamp(Object.Width / Object.ImageWidth + YourDeltaScaleValueHere, 0.01, 100) * Object.ImageWidth
I would use clamp to ensure your sprite doesn't either disappear or invert.
NB, if you have stretched or changed the dimensions of your object in the editor then that will have altered the scale such that this might not work as you anticipate; in such a case I would create a Scale family and, On Scale Created, set Scale.StartWidth and Scale.StartHeight values to the Width and Height values (to equate to a scale of 1). Then substitute StartWidth in place of ImageWidth etc in the equation above.
I hope I have managed to explain this ok? It's hardly simple, but you could put it into a function and call it whenever you need to, having only to write it once.