What is the wisdom behind the righteous man releasing his cow into the pastures instead of keeping it for his son and his mother, even though he knew his family would need it, and what is the meaning of reliance upon God (tawakkul) in this context?

This story is from the Israeiliyat (ancient tales of the Children of Israel) which are recounted for their anecdotal value, not for deriving rulings or using as proof. Its purpose is to demonstrate the virtue of honoring one's mother and the effect of righteous parents in safeguarding their children. It narrates that a righteous man from the Children of Israel entrusted his heifer to Allah for his son before his death. The son grew up and was dutiful to his mother. His mother informed him about the heifer, which was called "Al-Mudhahhabah" (the Golden One), and asked him to invoke the God of Abraham, Ishmael, and Isaac to return it to him. The son went and found the heifer, which spoke to him by Allah's permission and told him that his dutifulness to his mother granted him great power. He took it to his mother, who asked him to sell it. Then an angel came in the form of a human to test the young man, offering him increasing prices, but the young man stipulated his mother's approval. In the end, the angel informed him that Moses (peace be upon him) would buy it for a murder that would occur among the Children of Israel, at a price of its hide filled with dinars. Thus, that very cow was the one Allah commanded the Children of Israel to slaughter.

Assuming the authenticity of this story, it falls under the category of miracles of the saints (karamat al-awliya'), and it does not contradict reliance on Allah and taking means. Sheikh Ibn 'Uthaymeen mentioned that these Israeiliyat are neither to be believed nor disbelieved, and that the apparent meaning of the Quran does not indicate them. Otherwise, they would have been transmitted for their encouragement of filial piety.