Reneging a postdoctoral offer from Canada due to Canadian work permit delays
Whether this specific Canadian professor would understand we, of course, cannot tell. Yet people tend to be understanding of delays due to bureaucratic visa issues, or things just not working out - especially if you communicate this clearly. I would suggest reaching out to that professor, laying out the situation and seeing if anything can be done. Perhaps they're willing to delay your start by a year, perhaps not. What you don't want to happen is the professor reaching out to you a month from now, asking when you're planning to show up, and you having to respond that actually you started a completely different position.

A sidenote is that you'd probably not be legally bound to a one-year commitment in the US, but I think you're right in not wanting to burn those bridges before they're even built.