Repentance of the Thief is Acceptable

"But whosoever repents after his crime and does righteous good deeds (by obeying Allah), then verily, Allah will pardon him (accept his repentance). Verily, Allah is Oft-Forgiving, Most Merciful.
Know you not that to Allah (Alone) belongs the dominion of the heavens and the earth! He punishes whom He wills and He forgives whom He wills. And Allah is Able to do all things." (Qur'an 5 : 39)

Forgiveness on the part of Allah does not mean that the hand of the thief should not be cut off. It means rather that one who repents and becomes righteous by purging his soul of the sin of stealing will be spared the wrath of God, Who will remove the stain of that sin from him. But if after his hand has been cut off the person concerned does not purge himself of evil intent and continues to nurture the same impure feelings which led to his stealing and thus to the cutting off of his hand, it is evident that even though his hand has been severed from his body, stealing remains ingrained in his soul. The result will be that he will continue to merit God's wrath as he did before his hand was cut off. The Qur'an therefore directs the thief to seek pardon from God and to try to reform himself. For the hand of that thief was cut off for the sake of the judicious administration of human society and the cutting off of a hand did not automatically purify the soul of the person on whom the punishment was carried out. Purity of soul can be achieved only by repentance and turning oneself to God. Traditions mention that after the hand of a thief had been cut off in compliance with the Prophet's order, he was summoned by the Prophet (peace be on him) himself who said to him: 'Say: "I seek pardon from God, and to Him do I turn in repentance.'" The thief uttered these words as directed by the Prophet (peace be on him) who then prayed for the thief, saying: 'O God, accept his repentance.' (Abu Da'ud, 'Hudud', 8 - Ed.)

Therefore, whoever repents and goes back to Allah after he commits theft, then Allah will forgive him. Imam Ahmad recorded that `Abdullah bin `Amr said that a woman committed theft during the time of the Messenger of Allah and those from whom she stole brought her and said, "O Allah's Messenger! This woman stole from us.'' Her people said, "We ransom her.'' The Messenger of Allah said,

(Cut off her hand.) They said, "We ransom her with five hundred Dinars.'' The Prophet said,

(Cut off her hand.) Her right hand was cut off and the woman asked, "O Messenger of Allah! Is there a chance for me to repent'' He said,

(Yes. This day, you are free from your sin just as the day your mother gave birth to you.) Allah sent down the verse in Surat Al-Ma'idah,

(But whosoever repents after his crime and does righteous good deeds (by obeying Allah), then verily, Allah will pardon him. Verily, Allah is Oft-Forgiving, Most Merciful.) This woman was from the tribe of Makhzum. Her story was narrated in the Two Sahihs from Az-Zuhri from `Urwah from `A'ishah, The incident caused concern for the Quraysh after she committed the theft during the time of the battle of the Conquest ﴿of Makkah﴾. They said, "Who can talk to Allah's Messenger about her matter'' They then said, "Who dares speak to him about such matters other than Usamah bin Zayd, his loved one.'' When the woman was brought to the Messenger of Allah , Usamah bin Zayd talked to him about her and the face of the Messenger changed color (because of anger) and he said,

(Do you intercede in a punishment prescribed by Allah) Usamah said to him, "Ask Allah to forgive me, O Allah's Messenger!'' During that night, the Messenger of Allah stood up and gave a speech and praised Allah as He deserves to be praised. He then said,

(Those who were before you were destroyed because when an honorable person among them would steal, they would leave him. But, when a weak man among them stole, they implemented the prescribed punishment against him. By Him in Whose Hand is my soul! If Fatimah the daughter of Muhammad stole, I will have her hand cut off.) The Prophet commanded that the hand of the woman who stole be cut off, and it was cut off. `A'ishah said, `Her repentance was sincere afterwards, and she got married and she used to come to me so that I convey her needs to the Messenger of Allah.'' This is the wording that Muslim collected, and in another narration by Muslim, `A'ishah said, "She was a woman from Makhzum who used to borrow things and deny that she took them. So the Prophet ordered that her hand be cut off.'' Allah then said,

(Know you not that to Allah (Alone) belongs the dominion of the heavens and the earth!) He owns everything and decides what He wills for it and no one can resist His judgment,

(He forgives whom He wills and punishes whom He wills. And Allah is able to do all things.)