The conditional promise
דבר נא באזני העם וישאלו וגו’ כלי כסף וכלי זהב
Please tell the people to borrow…silver and gold vessels 
Just before the Exodus was about to take place, Hashem made an unusual request of Moshe. He told him to please ask the Jewish people to borrow valuables from their Egyptian neighbors. The usage of the word “please” indicated to Chazal that there was some special purpose to this request. They explain that Hashem was concerned (so to speak), that that righteous one, meaning Avraham, will have complaints against Him. Hashem gave Avraham a prophecy that his children will be strangers in a strange land. They will be oppressed and enslaved. However, the consolation is they will leave Egypt supremely wealthy.
As the Jews were about to leave Egypt, the first half of the prophecy had come true. However, leaving as they were, the second half wouldn’t be fulfilled. Hashem was concerned that Avraham would complain that his descendants were indeed oppressed, but they didn’t leave with great wealth. Therefore, Hashem asked Moshe to take special care that the Jews “borrow” valuables from the Egyptians, taking them with them. The obvious question on this Midrash is that why is Hashem only concerned with Avraham’s complaints? Shouldn’t Hashem want to be true to His word, even if Avraham wasn’t holding Him accountable?
What was the reason that Hashem specifically promised wealth to Avraham’s descendants? There’s a teaching that anyone who fulfills the Torah in poverty, will merit to fulfilling it in wealth. The problem is we see many instances where this doesn’t hold true. The true intent of this teaching is that wealth is a much harder test than poverty. Someone who has nothing will have no difficulty turning to Hashem. However, someone with wealth, is prone to forget their creator. They feel self-sufficient, and assume their lot was their own doing. These thoughts are very difficult to overcome. Hashem was informing Avraham that his descendants will overgo a period of enslavement. They will be worse off than any poor person. If they can manage to maintain their faith in such conditions, there’s hope they’ll do the same while in a state of wealth. They will be able to withstand that future test.
However, our sources tell us that the Jewish people forgot Hashem while in Egypt. The bondage of slavery was too intense for them, and they felt like they had no choice. If they continued to be Jews, their oppression would continue. Therefore, they annulled their covenant of circumcision, in order to appear more like the Egyptians. They were trying to assimilate. However, their plans didn’t succeed; the Egyptians continued their slavery. What’s worse, is they showed Hashem that they wouldn’t be able to withstand the test of wealth. They therefore lost the right to leave Egypt with immeasurable wealth. Hashem had no future intention of giving it to them. However, there was one problem.
Avraham was promised that his descendants would leave Egypt wealthy. If Hashem didn’t fulfill his promise, Avraham would complain. Why would he though? The Jews lost their right to the promise of wealth. They showed that they couldn’t withstand the test! Surely Avraham would have realized that. Perhaps this is why the Midrash refers to Avraham as that righteous one. This could be referring to his attribute of loving kindness. What makes Avraham stand out from other righteous people is his inability to see faults in others. We see this by his plight to Hashem that He spare the wicked city of Sedom. We see this with his anguish at expelling his wicked son Yishmael. He thought that they had done no wrong.
So too with Avraham’s descendants. If Avraham would be informed in the next world what had happened to his descendants, including their enslavement and subsequent freedom, he would be bewildered. He was promised that his descendants would leave Egypt incredibly wealthy. How could Hashem ignore His word? Even though the Jews had lost their right to the wealth, Avraham wouldn’t have seen it that way. He would have judged them favorably, and seen no fault. This would cause him to complain to Hashem for the way He was behaving. In order to avoid that, Hashem requested from Moshe that he have the Jews borrow valuables from the Egyptians. This way, they would leave with the promised wealth.
 Based on Kol Ben Levi § 23, by the Aruch HaShulchan (found at the end of Yoreh Deah)
 Exodus 11:2. Translation based on Rashi ad. loc. Cf. Targum Onkelos ad. loc.
 Berachos 9a, brought by Rashi ad. loc.
 Genesis 15:13,14
 Avos 4:9
 See Rashi to Deuteronomy 1:1, quoting Berachos 32b, and Deuteronomy 32:15
 Shemos Rabbah 1:8; Midrash Tanchuma Shemos § 5; Vayikra Rabbah 23:2
 See Micha 7:20, Zohar I p. 213b, Otzar HaMidrashim Atzilus p. 68,69, Kallah 9:6, Kesubos 8b, Tanchuma Yashan Chayei Sarah § 2, and Ramban’s HaEmunah VeHaBitachon. Cf. Ibn Ezra and Targum Yonasan ad. loc.
 Genesis 18:23-33
 Ibid 21:10,11
 Bereishis Rabbah 49:9 and 53:12, based on Isaiah 33:15
 What the Aruch HaShulchan doesn’t explain is how could Hashem put the Jews in a situation where they’d likely fail a test, just so Avraham wouldn’t complain