I just found it odd that Borusa promised the Master a whole new set of regenerations when he himself was searching for immortality. Well, one could argue that Borusa was searching for a method to immortalize his current incarnation, the one that obviously "fell to the dark side". I mean, when we first met him in "The Deadly Assassin", he was a bleedin' ponce. When we saw him again in "The Invasion of Time", he was a world weary sage. He was actually likable, at least, one felt he deserved respect. But the exchange between Davison's Doctor and his (for far) latest incarnation suggested Borusa wanted that specific body and personality to live forever.
I still feel that Borusa gave the Master a "snow job" about a new cycle of regenerations. If it was that easy, why did the Master need to concoct such an elaborate plan to access the Eye of Harmony when he looked like a deep fried druid? I read that "offer" scene as something like, "Your offer of a new cycle is such a bold faced lie, but I'll play along to discover your true objective. Whatever you're wanting, I suspect I'll want more."