Is it possible to lock the scale when entering dimensions? I have a Grid Floor with an area of one square kilometer and an origin at (0,0,0).

I would like to add a wireframe cube centered at (0,0,0) with a with dimensions 250 m by 250 m by 250 m.

On the Transform panel, when I lock the Location at (0,0,0) and lock the Scale at (1,1,1) and set the dimensions, the scale; although locked changes to (250,250,250).

Is there any way to lock the scale at (1,1,1)?
There are two alternatives:

Scale your object in Object Mode as you describe, and then Apply the scale (Ctrl+A). Your object will then have the correct dimensions and the scale will be reset to 1,1,1

As an alternative create your object and do all of the scaling in Edit Mode, that will not affect the scale of the object.