
Somebody accidentally broke his Itikaaf by leaving the Masjid. Is it possible for him to do his Qazaa the very next day?

Yes, he may sit another 24 hours with fasting during the remaining days of Ramadhan to complete his Qadha’.
It is also a personal recommendation for him to continue remaining days in Nafil I’tikaf and hope for rewards from Allah.
And Allah Ta’āla Knows Best
Mufti Faisal al-Mahmudi