To whom does the pronoun "ha" (هاء) refer in Sufyan al-Thawri's statement, "I am innocent of it if Abu Ja'far enters it," and is attributing kufr (disbelief) to something considered kufr?

It is not permissible to declare disbelief (kufr) when disbelief is made conditional on something, as this style may be used for emphasis and certainty. For example, when someone says: "I disbelieve in God if the Qur'an is not His word," he only intends to assert with certainty that the Qur'an is indeed the word of God Almighty. An example of this is the Almighty’s saying: "Say, 'If the Most Merciful had a son, then I would be the first of His worshipers'" [Al-Zukhruf: 81]. This is not an affirmation of the possibility that Allah, glory be to Him, could have a son, but rather an affirmation and confirmation of the negation. The majority of scholars have held that if someone says, "I disbelieve in God if I do such-and-such," and then does it, he is not required to offer expiation and does not become a disbeliever unless he harbored that intention in his heart. The statement by Imam Sufyan al-Thawri: "I disavow myself before Allah from Abu Ja'far if he enters Makkah," does not intend disavowal from Allah, but rather is an oath taken before Allah, showing persistence and expressing certainty in the acceptance of his supplication.