In order to use the features of ROM Manager you have to be rooted.
In order to install a custom ROM, you need an alternate recovery, which requires that you be rooted in order to install it.
IOW, you'll need to root first.
Also, I'm really not sure what you were trying to accomplish by unrooting to get the FRG83D (aka FroYo 2.2.1) if you were going to instal Sapphire anyway. When you install alternate ROMs, you (should) wipe all the partitions anyway, so, in effect, you just performed a lot of very unnecessary steps.
The ROM is composed of the stock parts ( /system and /boot) and your apps and cache are added in as /data and /cache. All of these you (should) wipe when installing a new ROM, so, in effect, you unrooted (which means now you have to root again), installed a stock ROM (which will be erased completely), and wasted a lot of time.
What was your rationale in doing ask of this instead of just doing as mentioned above?
(I am not trying to be sarcastic, I am genuinely curious as to why you felt the need to do all this...