He attempted to ambush Hijikata by pretending to surrender the CSS Stonewall to the Republic of Ezo. Also, he had to transport Yojiro to Ezo unharmed as a favor from Saigo.
To capture Hijikata, he pretended to hand over the CSS Stonewall to the Republic of Ezo. Additionally, he had to transport Yojiro safely to Ezo as a favor from Saigo.