Ākūti had two brothers, but in spite of her brothers, King Svāyambhuva Manu handed her over to Prajāpati Ruci on the condition that the son born of her be returned to Manu as his son. This he did in consultation with his wife, Śatarūpā.
Sometimes a sonless person offers his daughter to a husband on the condition that his grandson be returned to him to be adopted as his son and inherit his property. This is called putrikā-dharma, which means that by execution of religious rituals one gets a son, although one is sonless by one's own wife. But here we see extraordinary behavior in Manu, for in spite of his having two sons, he handed over his first daughter to Prajāpati Ruci on the condition that the son born of his daughter be returned to him as his son. Śrīla Viśvanātha Cakravartī Ṭhākura comments in this connection that King Manu knew that the Supreme Personality of Godhead would take birth in the womb of Ākūti; therefore, in spite of having two sons, he wanted the particular son born of Ākūti because he was ambitious to have the Supreme Personality of Godhead appear as his son and grandson. Manu is the lawgiver of mankind, and since he personally executed the putrikā-dharma, we may accept that such a system may be adopted by mankind also. Thus, even though one has a son, if one wants to have a particular son from one's daughter, one may give one's daughter in charity on that condition. That is the opinion of Śrīla Jīva Gosvāmī.
(Srimad Bhagavatam-----4:1:2-----purport).

The Supreme Personality of Godhead is self-sufficient, but to enjoy His transcendental bliss He requires the cooperation of His devotees. In Vṛndāvana, for example, although Lord Kṛṣṇa is full in Himself, He wants the cooperation of His devotees like the cowherd boys and the gopīs to increase His transcendental bliss. Such pure devotees, who can increase the pleasure potency of the Supreme Personality of Godhead, are certainly most dear to Him. Not only does the Supreme Personality of Godhead enjoy the company of His devotees, but because He is unlimited He wants to increase His devotees unlimitedly. Thus, He descends to the material world to induce the nondevotees and rebellious living entities to return home, back to Godhead. He requests them to surrender unto Him because, unlimited as He is, He wants to increase His devotees unlimitedly. The Kṛṣṇa consciousness movement is an attempt to increase the number of pure devotees of the Supreme Lord more and more. It is certain that a devotee who helps in this endeavor to satisfy the Supreme Personality of Godhead becomes indirectly a controller of the Supreme Lord. Although the Supreme Lord is full in six opulences, He does not feel transcendental bliss without His devotees. An example that may be cited in this regard is that if a very rich man does not have sons in a family he does not feel happiness. Indeed, sometimes a rich man adopts a son to complete his happiness. The science of transcendental bliss is known to the pure devotee. Therefore the pure devotee is always engaged in increasing the transcendental happiness of the Lord.
(Srimad Bhagavatam-----9:4:64-----purport).

Kṛṣṇa's rasa-līlā means that Kṛṣṇa was adopted or foster child. Is that foster child? Yes. So, of Nanda Mahārāja. His father exchanged. His father Vasudeva wanted to save Kṛṣṇa, and he brought from Mathurā to Vṛndāvana, Gokula. And Nanda Mahārāja had a daughter. So he placed Kṛṣṇa on the lap of Yaśodā and took away the daughter, and this daughter was given to Kaṁsa for killing. Just see, there is selfishness. He wanted to save his own son and wanted to kill Yaśodā's daughter. (chuckling) You see? So this is not selfishness; therefore we shall misunderstand. That daughter was not ordinary child, she was the material energy herself, Durgā. So when Kaṁsa took that child by the leg and wanted to, what is called? Slash? Smash, yes. Smash on the stone, the daughter slipped from his hand and appeared as Durgā, that "You fool, you want to kill me. The boy, the child who will kill you, is already born. Don't try for killing me. He's already born." So Kṛṣṇa, up to sixteen years, He was in Vṛndāvana.
(General Lecture).