I've done National Free Day one-ways from the US to Canada as recently as a few weeks ago (start
here and read my discussion with
@myi) but it seems like National is blocking one-ways from BUF to YYZ right now and nothing showed online when I looked for someone earlier this week. Hertz was showing only when logged into my PC account but I needed to book for someone else so I figured I'd try I booking and then calling to change the name but when I called they said one-ways from BUF to YYZ are blocked and even though the website let me book online it wasn't valid. Not sure what would've happened if I just showed up but the phone rep was adamant that they'd either not give me a car or they'd give it to me but YYZ wouldn't accept it.
No experience with DTW but I'd have to imagine that it's similar.
What might work better is trying from a city that's further from the border, like NYC. It's a longer drive but maybe they're not blocking one-ways (unless they're blocking anything cross-border).
TLDR: AFAIK you might not be able to find a one-way at all, much less a one-way with a good discount code.