Explanation

These readings reflect linguistic options for the pronoun attached to this preposition. While the norm (including for Ḥafṣ) is to pronounce this with kasrah due to the preceding ya', the change here may be to produce the heavy sound in the divine name, which reflects the gravity of the pledge. The default of the pronoun itself is -hu, so the reading of Ḥafṣ may also reflect the principle of sticking firmly to the original pledge [al-Alusi].