Bulugh al-Maram

Takhrij:
«أخرجه البخاري، الدعوات، باب قول النبي صلي الله عليه وسلم: ((اللّٰهم! اغفرلي...))، حديث:6398، ومسلم، الذكر والدعاء، باب التعوذ من شر ما عمل...، حديث:2719.»©Explanation:
➊ All such supplications that have been authentically reported from the Prophet sallallahu alayhi wa sallam, he made them in compliance with the divine command, because he was infallible from error; or he made them with the intention of teaching the ummah.
➋ In some narrations, it is mentioned that the Prophet sallallahu alayhi wa sallam would recite this supplication at the end of the tashahhud, and in others, it is mentioned that he would recite it after concluding the prayer with salam.
It is entirely possible that he recited this supplication in both ways—sometimes before the salam, and sometimes after the salam.

Hadith Commentary:
(1)
Regarding the exalted status of the Messenger of Allah (sallallahu alayhi wa sallam), Allah the Exalted has stated:
“It is expected that your Lord will raise you to a praised station.” (: Bani Isra’il: 79)
He has also said:
“Surely your hereafter will be far better for you than the present, and your Lord will soon grant you so much that you will be pleased.” (: Ad-Duha 4-5)
Since supplication (du‘a) is an act of worship, indeed the very soul of worship, the Messenger of Allah (sallallahu alayhi wa sallam) made such supplications.
Furthermore, you made these supplications either as an expression of servitude or to teach the ummah.
These supplications were not made on the basis that you were actually sinful or in error.
Undoubtedly, you (sallallahu alayhi wa sallam) were infallible from sins and free from disobedience, as the Noble Qur’an itself has explicitly stated. (: Al-Fath: 9)

(2)
Hafiz Ibn Hajar (rahimahullah) has written that the Messenger of Allah (sallallahu alayhi wa sallam) would recite these supplications during the prayer, before the salutation (salam).
Some narrations indicate that they were recited after the prayer.
In light of these narrations, there is a strong possibility that the Prophet (sallallahu alayhi wa sallam) would recite these supplications both before and after the salutation.
(: Fath al-Bari: 11/236)

Hadith Commentary:
Saying the words "wa anta ‘ala kulli shay’in qadeer" ("and You are powerful over all things") at the end of the supplication is an expression of the fact that Allah, the Exalted, is capable of everything; He can do whatever He wills and is in need of no one. This divine self-sufficiency is such that even the greatest prophets and the most favored servants tremble before it, and day and night, with utmost humility, continue to confess and acknowledge their shortcomings. If even a trace of self-conceit enters anyone’s heart, then there remains no place of refuge for him.

Hazrat Shaykh Sharfuddin Yahya Manbari rahimahullah states in his letters that the Pure Lord is so self-sufficient and independent that if He wills, He can create millions of people every day like Ibrahim and Muhammad sallallahu alayhi wa sallam, and if He wills, He can, in a single moment, turn all the favored servants into those rejected from His court.

Jalla Jalaluhu.

Here, the mention is of will (mashi’ah); will is one thing and law (qanun) is another thing.

Regarding the divine laws, there is a clear statement:

﴿فَلَنْ تَجِدَ لِسُنَّتِ اللَّهِ تَبْدِيلًا وَلَنْ تَجِدَ لِسُنَّتِ اللَّهِ تَحْوِيلًا﴾ (Fatir: 43)
Allah the Blessed and Exalted has spoken the truth.