Genesis 21:17

"The voice of the boy" - From here, [we learn that] the prayer of a sick person is better than the prayers of others on his behalf, since the prayer continues to be received.

"From where he was" - (Rosh HaShanah 16) - Based on the actions that [Ishmael] is doing now, he is considered, and not according to what he will do in the future. This is because the Angels accuse him and say "Creator of the Universe! In the future his offspring will kill your sons through thirst, and you're giving him a well [to drink from]? And [G-d] replies, "now is he a righteous person or a wicked person?" [The Angels] say, "he's a righteous person." (In other words, regarding this matter, [Ishmael] isn't fitting to receive a punishment of death through thirst). [G-d] said to them, "according to the actions of a person now do I judge them....

And the voice of the youth was heard before the Lord for the righteousness' sake of Abraham; and the Angel of the Lord called to Hagar from heaven, and said, What to thee, Hagar? Faint not, for the voice of the youth is heard before the Lord; neither shall judgment be according to the evil which he will do, but according to the righteousness of Abraham is mercy upon him in the place where he is.

WHERE HE IS. He shall be judged according to his present deeds, and not according to those actions which he may do in the future. This was because the ministering angels laid charges, etc. Thus the words of Rashi quoting from the teachings of our Rabbis., 19.
The correct interpretation, in line with the simple meaning of Scripture, appears to me to be that the verse is stating that G-d heard the voice of the lad in the place in which he was. He informed her that she will not need to go from there to a fountain or well for in that very place he will quench his thirst immediately. He thus said to her, “Arise, lift up the lad after you will have given him to drink, for I will make him a great nation. Similarly, the word sham (there) in verses, Where he sunk, there he fell down dead; And where the slain are, there is she, alludes to the place.

באשר הוא שם, seeing she could not see him from the place where she had sat down, the angel had to use this manner of referring to Ishmael. He would be provided with water at that location.

וישמע אלוקים את קול הנער, he too had raised his voice and was crying.

מן השמים, she heard his voice without seeing the source of the voice as opposed to the first encounter she had with an angel in 16,13. At the beginning of the verse the Torah uses the proposition את, whereas at the end of the verse it switches to the preposition אל. The letters ת and ל are used interchangeably here without the meaning of the preposition undergoing any change.

באשר הוא שם, at the very site where Hagar had thrown the boy. The message to Hagar was that at the very site where she thought that her so was going to die, there G’d heard his voice, his prayer. Our sages (Rosh Hashanah 16) derive from this wording that human beings are not judged by the Heavenly Court on the basis of what they might become guilty of in the future, but their judgment is based on their status at the time judgment is in progress. If someone is wicked at the time the Heavenly Court is in session, the fact that G’d foresees that he will (would) become righteous at some point in the future is not taken into consideration. Equally, if someone at the time when the Court is in session is deemed innocent, the fact that G’d foresees that he will become a great sinner is not used to influence his judgment at that time. There is an interesting aggadah, according to which the angels remonstrated with G’d at this time, challenging G’d’s mercy on the basis of the untold harm Ishmael’s descendants would visit upon G’d’s people, the Jews in the future. The angels referred specifically to the Ishmaelites denying Jews on the march into exile water to quench their thirst. (a reference to Isaiah 21,13-14) G’d silenced them by asking whether at this particular point in time Ishmael deserved to die. They had to admit that he did not. Hence G’d felt free to exercise His mercy and to let him live.