Is it permissible to append the intention of not returning to a sin with the phrase "if Allah wills," by saying, "If Allah wills, I will not return to such and such a sin"?

People's words are interpreted based on their intentions. The phrase "إن شاء الله تعالى" (if God wills) can imply doubt, or certainty while making an action conditional on God's will, or seeking blessings by uttering it. If the intention behind saying "I will not return to sin, إن شاء الله تعالى" is doubt and hesitation in one's resolve, then this is not valid; because a condition for sound repentance is a firm resolve to abandon the sin. However, if the intention is to seek blessings or to make the matter conditional on God's will without hesitation or doubt, then this is permissible, and it falls under the general meaning of His Almighty saying: (And never say of anything, "Indeed, I will do that tomorrow," Except [by adding], "If Allah wills") [Al-Kahf 18:23-24], as it involves referring the matter to God's will and seeking His help.