Turning down an invitation
For some reason I got it into my head that
すみませんがちょっと was a way to turn down an invitation.
But for example say you were asked to do X at time Y, and you want to do X but time Y doesn't work for you. You can say
よじはちょっと: in this case "4 o' clock is a little bit inconvenient".
Why does one use the particle が and the other the particle は? Or am I just completely incorrect and it should be
すみませんはちょっと all along?