All versions above are bollocks.
Here's what happened:
Roger's voice was exhausted from singing "Shine On You Crazy Diamond" and getting his lead vocals the way he wanted them. It was Roger who asked Roy Harper to sing the song; Roy was recording an album in another studio at Abbey Road. Dave had already worked with him on this same album.

And all this stuff about Dave not wanting to sing "Have a Cigar" because of the "negative" lyrics is also bollocks: HAC has the same exact cynicism as "Money" or "Not Now John" and Dave had no problem singing those.