In the film, Asterix is drugged and hypnotized into believing Obelix is Caesar resulting in a fight and causing a distraction for Prolix and his associates to steal the money and escape.
In the film, Asterix becomes disoriented due to medication, leading him to believe Obelix is Caesar, which causes a commotion and allows Prolix and his associates to take advantage of the situation to steal the money and make their escape.