There's an author of folklore and the paranormal called Varla Ventura who's said in old British and Scandinavian folklore trolls are the guardian spirits of bridges over water. I believe the toll, or originally an offering was meant as thanks for safe passage. Not sure on the original sources but I've heard her mention it in podcasts a couple of times over the years.

This offering could possibly later be getting conflated with paying a toll to cross a bridge.