Complex sentence: Cahn personally auditioned the song for Astaire's approval which he did approve; however, Astaire's contractual obligations later prevented him from making the film and the role would go to Jackie Gleason who introduced the song.
Split into simpler sentences: Cahn personally auditioned the song for Astaire's approval which was given. However, Astaire's contractual obligations later prevented him from making the film and the role went to Jackie Gleason, who introduced the song.