He doesnt need to sacrifice himself.
He got like a billion Sharingan´s in his hideout..just pop his own MS out, insert a random-fodder-Sharingan, activate Izanagi, use Rinne Tensei, come back to life and insert his own MS again.
Easy as that, its just that Obito doesnt want to help Madara because Madara is most likely following a completly different plan.