We play on the sand and there is a dispute as to how to judge a ball in or out. If the ball hits the line, so that the line clearly moves, but then strikes the sand an inch out of bounds, is the ball in or out by official rules? This happens all the time and we have always played "line is in" but recently many want to change to "go by the mark in the sand". Which is correct? Due to the contentious nature of this, I'd appreciate reference to official rules and standards with citations, more than personal opinions or anecdotes.
The FIVB Beach Volleyball rules are very clear on this. Quoting from Rule 8.3, Ball "in":
The ball is "in" when it touches the surface of the playing court including the boundary lines.
See also Section 5.1.2 in the 2015 Casebook which pretty much covers your situation precisely:
Referees should be aware that the ball may land out, but has struck the line, which is in a raised position. In this case, the ball contacts the line and must be called "in".