This is the same topic that came up a year or two ago when there were rescheduled games due to COVID. BetNow was caught freerolling customers on teasers and someone with optional brains across the street could not understand it, and so would not stick up for the player.
Books can do many fair things. #1 is the easiest.
1. Cancel any/all teasers involving the Bills game. In your example, that means canceling yours as well as anyone who bet against the 49ers on Sunday. I believe Heritage does this. BetNow, however, was grading Chargers/Bills teasers as losses and 49ers/Bills teasers as pushes. BS. The player is in a no-win situation just because of a rescheduled game.
2. A two team teaser is basically a 2 team parlay where the player has bought points on both teams. If you know the fair value of each leg, then 49ers/Bills tickets could be paid out as one leg straight bets. The problem is there's no stated odds on what 49ers -1.5 should've paid. If books changes their software to start booking teasers as multi team parlays with bought points, then this would be do-able.