Keep this on topic and that counts especially for you H3llScreaM. You start attacking him while you clearly didn't read his thread carefully. Also, as already been said, the manga doesn't say anything about Danzo loosing his eye and arm in a battle against Sarutobi, it doesn't even say they fought each other at all.
What they did say is that they both competed for the title of hokage. End of discussion.

On topic I just think he lost his eye in combat and later transplanted the Sharingan. This way no one would actually be suspicous about his eye as it already was gone.