Obito: He approached me and made an offer. In return for allowing me to get revenge on the Uchiha he asked me not to harm the rest of the village. I said I would help annihilate my family. However, the 3rd Hokage had other plans. He tried to reach a truce with the Uchiha. But time pressed on and he failed.
Like I said the 3rd didn't want to destroy the Uchiha. Also based on this we can create a timeline since talks with the Uchiha to prevent war were still taking place. Meaning the final order to destroy the Uchiha was never given when Obito & Itachi first met. Sushui lost his eye in an effort to sway Danzou's decision on destroying the Uchiha. Meaning some time had past between Itachi & Obito's meeting & the actual massacre in question. Since Sushui died just days before the massacre Itachi couldn't have had MS when he first met Obito since he met him some time before Sushui's actual death. You can STFU now.