The Redskins were without top two quarterbacks Robert Griffin III and Kirk Cousins due to injury, however third-stringer Rex Grossman fired them into the lead as he threw a seven-yard touchdown to receiver Pierre Garcon.
Doug Marrone's side hit straight back with a two yard run from CJ Spiller to tie the scores. However, the Bills were hit with a double injury woe as Spiller and quarterback Kevin Kolb were forced to leave the game.
Forbath added two more field goals before running back Keiland Williams sealed the win in the fourth quarter with a one yard run.
Matters got worse for Buffalo as starting cornerback Stephon Gilmore also left the game with a wrist injury as the Redskins eased to their third win of pre-season.