It is permissible for him to intend to complete the prayer on his own and leave while the Imam is still praying if there is an emergency or he has to go for a legitimate reason.
This was done at the time of the Prophet salla Allahu alaihi wa sallam when Zaib Ibn Thabit used to lead the prayer and prolong it till one of his companions left the prayer and continued praying alone and he left the congregation.