Just so I'm clear, is the discoloration where the new grout was applied? If so, and the dust is in the grout, you need to replace the grout. Clean area well first.

If the discoloration is in the surrounding areas, a good grout cleaner and a brush to agitate should get it up.

Photos please. And is there a reason why the tile setter isn't doing this type of work for you?

I used Fusion Pro on some soapstone tiles. It left some stains on the tiles that nothing would remove, except sandpaper. The sanding dust got into the grout and nothing would remove that either.

Solution: Go over the discolored grout with new grout, then wipe it all off. The dye in the grout should bring that old grout back to the right color.