Why did G-d "relent" to Isaac and Rivka's prayers?

You'll find we've already written a Dvar Torah that explains a Pasuk in Toldos, located here

This Dvar Torah blew me away. When it was clear that they would be unable to have children, Isaac and Rivka prayed, and the Pasuk (21:25) says:

'וַיֶּעְתַּר יִצְחָק לַיהוָה לְנֹכַח אִשְׁתּוֹ, כִּי עֲקָרָה הִו וַיֵּעָתֶר לוֹ ה - "And Isaac entreated the Lord for his wife, because she was barren; and He relented to him"

A gentleman once told R' Yosef Chaim Sonnenfeld a short Dvar Torah on this. Why does the Pasuk say "and He relented to him". Does this not seems an odd way of saying that G-d heard and heeded his prayer?

The gentleman explained that there is a tradition that all the Patriarchs were meant to live until the age of 180. Yet Avraham died aged 175 (25:2). We can work this out: Avraham was 99 when he circumcised himself, 100 at Isaac's birth, 137 at the Akeida, at which point Isaac was obviously 37. Isaac was 40 at his marriage, and 60 at the birth of Yaakov and Esau (25:26) making Avraham 160 at their birth. Esau first murdered aged 15, making Avraham 175. There is a Rashi that Avraham would live to see Yishmael repent (25:9), but die before Esau killed someone for the first time (25:30). If he died aged 175, where are the missing 5 years?

The gentleman said to R' Yosef Chaim Sonnenfeld that 'וַיֵּעָתֶר לוֹ ה is the answer. There was never any doubt that Isaac would have children, his father had been promised "nations", certainly Isaac would have children then, and Rivka would certainly be their mother as she was the one deemed worthy, as evidenced by the miracles Eliezer witnessed. וַיֵּעָתֶר לוֹ ה can be interpreted to mean that G-d did not want to give them children just yet, He simply wanted them to wait a bit longer so Avraham would not live to see his grandson become a murderer.

When R' Yosef Chaim heard this, he jumped up and exclaimed that this vort was Emess, because 'וַיֵּעָתֶר לוֹ ה is Gematria (numerical value) 748 (686+36+26), which is the same Gematria as חמש שנים - five years - 748, which represents G-d relenting to their prayers to have children at the expense of חמש שנים from Avraham Avinu's life.

R'Yosef Chaim also said that we say " רְצוֹן-יְרֵאָיו יַעֲשֶׂה; וְאֶת-שַׁוְעָתָם יִשְׁמַע, וְיוֹשִׁיעֵם - He will fulfil the desire of them that fear Him; He also will hear their cry, and will save them ". Isn't this in the wrong order? Doesn't Hashem fulfil their desires to placate their cries? R' Yosef Chaim explains that the prayers of a devout person are able to change what is meant to happen to people, which explains fulfilling their desires. But people don't know what's best for them, and sometimes suffer as a consequence of getting what they desire. The pasuk teaches that Hashem will even repair this later cry of suffering that is of their own doing.


Geshmack !