Wrote the song, played one of the best guitar solos ever, sang the lyrics he'd written, and left a space for the sax part, which was dependent on the chord progression and melodies around it.

The sax break on Pink Floyd's 'Money' wasn't written by any member of the band. The vocal line on 'Great Gig in the Sky' wasn't written by any member of Pink Floyd. There are so many instances of lines being added by folk who weren't 'part of the band'. Session musicians get used to it, but it helps if there's money involved.