I'm not Canadian, but I'd assume many US legal issues would be the same. Here, there may two things to consider. First, you're going to have to get a survey of both parcels before you do it. Not sure how much he's going to pay you for 20ft, but two survey would probably cost you at least $1,000. Second, you're going to have to run this by the municipality before you split because (a) they need to reset their tax legal descriptions and (b) you're going to have to file for a property split since you're breaking up platted parcels. You can probably have the surveyor help you with that, but you may need an attorney to take a peek as well.
If you're only going to make $10k or so off of this, I probably wouldn't do it. Not worth the hassle.