You have to know the seals to cancel the contract.
And maybe Kabuto didn't even know about this... maybe he was telling the truth. From what he knows there might not be any drawbacks of the jutsu. Otherwise Itachi could have found out about this, and being so smart he could have thought that maybe Madara knew it as well.
But in any case he didn't want to be here anymore. He lived his life, did what he thought best and his time was over, he already entrusted Sasuke to Naruto (again) and redeemed himself a little by stopping edo tensei.