My opinion...
I believe you've already met your ethical obligations by bringing it up the first time when they failed to deduct it. They indicated they would take care of it. Unless it's your job to ensure they do their jobs, you're done IMNSHO.
If you want to be covered in case they suddenly realize it later, take the amount of the commission they were supposed to have deducted and put it in a savings account or money market account, just in case they come back asking for it. If they don't, it was apparently their decision to leave it that way. Who are you to question them? You've got calls to make, after all...
