All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad 
  is His slave and Messenger. 
The Bedouin’s request to cancel his pledge can be interpreted as a request to cancel his conversion to Islam and accordingly would count as a declaration of disbelief. It could also be a request to cancel his pledge to make Hijrah by settling in Madeenah, which was required as one of the essential obligations of Islam at that time. This Bedouin might have wanted to ask the Prophet 
  to relieve him from the obligation to settle in Madeenah so that he could leave and go back to his family. Each of these two possible interpretations was adopted by a group of scholars. Al-Haafith Ibn Hajar 
  wrote, "The apparent indication of the Bedouin’s words is that he requested canceling his conversion to Islam. It could also be interpreted as a request to cancel one of the obligations of Islam, such as immigration to Madeenah as it was then a fundamental obligation and stern warnings were reported for whoever would leave Madeenah after migration..." [Fat-h Al-Baari]
Ibn Battaal 
  held that the preponderant view is that the Bedouin did not intend to request canceling his conversion to Islam. If he had been requesting to leave Islam, the Prophet 
  would have killed him for apostasy. Ibn Battaal wrote:
"The evidence that this Bedouin was not requesting the cancellation of his conversion to Islam is that he sought the permission of the Prophet 
  before canceling his pledge. If he intended his departure from Madeenah to be a declaration of his reversion to disbelief, then the Prophet 
  would have killed him for apostasy when he left. However, this Bedouin left Madeenah as a sinner, and the Prophet 
  excused him given his illness or his possible ignorance of the fact that immigration was obligatory on him, and he was hence considered one of those about whom the following verse was revealed (which means): {The Bedouins are stronger in disbelief and hypocrisy and more likely not to know the limits of what (laws) Allaah has revealed to His Messenger...}" [Quran 9:97] [Sharh Al-Bukhari] 
Al-Mubaarakfoori 
  wrote, "The Bedouin asked for canceling his conversion to Islam, and this opinion was decisively held by ʻIyaadh. Other scholars maintained that the Bedouin was seeking canceling his pledge to migrate; otherwise, the Prophet 
  would have killed him for apostasy. This means that the Bedouin did not want to give up Islam..." [Sharh Mishkaat Al-Masaabeeh]
As long as the hadeeth has different possible interpretations, it cannot be used as evidence that the apostate is not to be killed given that other explicit texts exist from the Sunnah and the reported scholarly consensus that decisively assert that the apostate must be killed. For instance, the Prophet 
  said, "Whoever changes his religion (i.e. leaves Islam), kill him." [Al-Bukhari and others] 
Ibn Qudaamah 
  wrote, "The scholars unanimously agreed that the apostate must be killed. This has been narrated on the authority of Abu Bakr, ʻUmar, ʻUthmaan, ʻAli, Muʻaath, Abu Moosa, Ibn ʻAbbaas, Khaalid, and others. None has been reported to argue otherwise; hence, it is a consensus." [Al-Mughni] 
Ibn Farhoon 
  wrote, "Al-Mutayti advised, 'As far as I know, the scholars unanimously agree that if a Muslim apostatizes, he is given three days to declare his repentance; if he does not do so, then he should be killed. However, only ʻAbd Al-ʻAzeez ibn Abi Salamah held that the apostate must be killed without offering him this three-day grace to repent.'" [Tabsirat Al-Hukkaam] 
Allaah knows best.