Is it permissible to recite Sayyid al-Istighfar (the master of seeking forgiveness) if a Muslim feels that he is not truthful in his saying: "And I am upon Your covenant and Your promise as much as I am able," and would he be considered sinful if he says that and does not remain faithful to the covenant?

Adherence to God's covenant and promise is crucial and serious. What is meant by God's covenant is His Oneness (Tawhid) and belief in Him, and it includes all religious obligations. Repentance is immediately required upon feeling any insincerity in that phrase, along with renewing the resolve to adhere to God's covenant and promise. Al-Khattabi said that the phrase "I am upon Your covenant and Your promise as much as I am able" means "I am upon what You have covenanted me upon regarding belief in You and sincere obedience to You." Ibn Battal also stated that the covenant is the solemn pledge God took from His servants when He brought them forth like ذر (tiny particles), and the promise is the admission of those who die upon Tawhid into Paradise. The condition of "as much as I am able" acknowledges incapacity and shortcomings in fulfilling the essence of the obligation. Breaching God's covenant varies in severity according to the nature of the transgression. A servant's insistence on non-compliance makes him sinful, whether he recites the supplication or not. The servant is commanded to fulfill the covenant whether he utters it verbally or not.