It's not! As the title says, it's a simple case of overbooking, and acceptance by the passenger of an offer to travel on any other service means that this is a case of voluntary denied boarding. EC261/2004 therefore does not apply.

When I took my first ever A3 flight last year (OSL-ATH) they asked me at check-in at the airport if I would like to take a later flight and accept €250; given that I was in business class and the later flight would be economy, and that I only had one day in Athens before flying onwards, I declined.

To the OP: If they ring back (and they may not, because other passengers contacted may have been more proactive in accepting the offer and cash immediately) they will detail the money transfer process.

You have no reason to fear that they won't pay up.