OK, I did try a minus sign already 😬 and no it doesn't work. I called in to your phone support line, and what ended up being the method to get around this was to go through the EasyStep interview process - several screens long, until you get a screen where you can enter or edit the amount on your T3 slip.  On that screen, it is possible to enter a minus sign for the amount in Box 42.  Took a bit of work, but leaving this resolution here in case others have the same issue.