…so that when you are decelerating, the ramjet works as before, but at the other end

Those atoms of dust (*) have mass, and you're now trying to accelerate them in the same direction the scoop is facing - same as flipping the entire vessel end-for-end. Unless I'm missing something basic(?)

If it was a magnetic scoop, It might work as some kind of sail, essentially a sea-anchor - but the dust it's gathering still has to go somewhere. And that leaves the issue of slowing when below the speed needed to accumulate hydrogen. If your intention was to head from star to star, maybe heading into the photosphere? Might be an interesting SF plot bit, anyway…