There is nothing in EU261 that says that the offer to
volunteers has to be the same as the statutory compensation for actually being subject to IDB.
Article 4.1
When an operating air carrier reasonably expects to deny boarding on a flight, it shall first call for volunteers to surrender their reservations in exchange for benefits under conditions to be agreed between the passenger concerned and the operating air carrier. Volunteers shall be assisted in accordance with Article 8, such assistance being additional to the benefits mentioned in this paragraph