Hashem speaks to Moshe to request from Bnei Yisrael  to please borrow from the Egyptians silver and golden vessels and Rashi adds the Chazal (Berachos 9) in order that Avraham will not have a claim against me that the first part of the decree of Bris Bein Habasarim which is the servitude to Mitzrayim You fulfilled, but the second part of leaving with a great wealth You did not execute.  Was collecting the riches of Mitzrayim for personal ownership such a difficult task that Hashem had to use the word Please?
Let us talk for a minute about Jewish possessions for it’s not what one would expect to think. In the plague of Dever where the Egyptian animals died, one passuk says וממקנה בני ישראל לא מת אחד וישלח פרעה והנה לא מת ממקנה ישראל. The first passuk uses the title בני ישראל in contrast to the second which uses ישראל omitting בני. The Tzemach Dovid explains that the first passuk relates that Pharaoh realized the truth that the plague did not affect any livestock from Am Yisrael. However, when he checked the Egyptian livestock if every single one died as prophesied he saw that many were still alive. He investigated further into the matter and discovered that the Egyptian animals that survived the plague were not originally offspring owned by the Egyptians but rather from the livestock that the Egyptians stole from Am Yisrael over the years originating from Yaakov and his children when the descended to Mitzrayim. This is what the second passuk says that even those animals that were offspring from the original flock of Yaakov and handed down stolen from one Egyptian generation to the other’ from they too not one died. To this group of livestock the passuk says והנה לא מת ממקנה ישראל not one of the livestock that originated from Yaakov’s (Yisrael) flock died even though they were repossessed by the Egyptians.
Look how much beracha and kedusha of a Yid’s possessions remain in tact even if they change hands and fall under different gentile owners all along the way! Pharaoh realized that not even one of these assimilated animals perished along with the Egyptian’s livestock during the plague of Dever.
The Igra D’Pirke 126 writes in the name of Menachem Mendel from Riminov to address a perplexing phenomenon that unfortunately repeats itself over and over again. One can find young innocent children who learn with such a fervor and enthusiasm yet when they grow older their conduct is filled with bad middos and they stop learning and davening. The Rebbe explained that their parents fed them with stolen money by not being straight in business, and food that is forbidden when eaten becomes the blood cells of the body and energy, eventually causing their bodies to be taken over by negative forces of evil. The spiritual fruits, that such a body will produce are compared to one that eats treifah foods which spiritually pollute the body. 
Even in the donations to the building of the Mishkan we find that Betzalel was given the gift to read the thoughts of the donor’s in order to know towards the making of which vessels their money should be given. Those who had purer and nobler thoughts were given to the making of the vessels in the Kodesh Kodoshem, and the Kodesh. Those donors who gave with less purity and motivation were used to build the courtyard. My Rebbi Rav Shlomo Freifeld told us that the Satmer Rav R Yoel ztl wore a bekeshe that had many pockets. All throughout the day his chassidim would give to the Rebbe donations. Each pocket was designated for the level and degree of kosher gelt. The highest level of purity would be directed to the feeding of the talmidim. The lower levels of giving would be directed to the physical maintenance of the schools and shuls such as paying the janitors.   
Rashi points out how stolen food by a bird cannot be brought up on the Mizbeiach and he writes (Vayikrah 1,16) the passuk says that a bird sacrificed as a Korban Olah, He shall remove its crop and innards and he shall throw it next to the Mizbeiach on the east to the place of the ashes. Regarding an Olah offering of an animal which eats from the feeding trough of its owner he should wash its innards and sacrifice them. But with regard to fowl which takes its sustenance from that which is stolen, he shall throw away the innards for it ate from that which is stolen.
We find a similar conduct by Avraham Avinu after defeating the Four Kings and the King of Sodom offered him the spoils of war. Avraham responded I swear I will not take even a shoelace from the spoils for once it becomes Sodom’s the booty is accompanied by the owner’s middos and thoughts of negative middos. Avraham refused to take a shoelace into his possession lest it would somehow compromise his spiritual purity and reasoning. The Shemen Rosh writes this is hinted in the passuk (Bereishis 14,22) אם מחוט ועד שרוך נעל ואם אקח מכל אשר לך I will not even take a string or a shoelace, for if I take anything then מכל אשר לך I am also taking with it everything from your inner being. It’s a package deal whether you like it or not.
For this very reason BneiYisrael would refuse to take over the wealth of Mitzrayim   culture steeped with decadence and waywardness lest it would affect their cleavage with Hashem. This is the reason why Hashem told Moshe to tell them please take this wealth of Mitzrayim in order to fulfill His promise given to Avraham at the Bris Bein Habasarim.
Now one might ask if Yisrael were so wary of taking the wealth of Mitzrayim why did they do it anyway. We must take note that Hashem said that Yisrael should take the money שלא יאמר אותו צדיק  so Avraham the tzadik will not say that I did not fulfill My entire promise in the Bris Bein Habesarim. Why is Avraham referred to with the title of tzaddik? It is for the reason to comfort the Yidden from their anxiety of taking the Egyptian’s wealth from fear of being influenced. Avraham refused to take from Sodom’s wealth in order to remain on and preserve the level of tzadik which he achieved. מעשה אבות סימן לבנים. Therefore if he insisted for you to yes take from the wealth from Mitzrayim that means he personally insured that you too will not be affected or influenced from this wealth to compromise your ruchniyus.
This can be the reason why the wealth that they took from Mitzrayim is called רכוש גדול and not רכוש רב. The term גדול  refers to Avraham Avinu. In Shemoneh Esrai we say  we say האל הגדול הגבור והנורא the title גדול refers to Avraham who was called האדם הגדול בענקים, ואעשך לגוי גדול, Yitzchak is the middah of גבורה and Yaakov is נורא מה נורא המקום הזה. The רכוש גדול was Avraham’s present to Klal Yisrael that their first beginnings concerning money as a nation should be possessed without lust and craving. Just the opposite, the tzadik Avraham orchestrated that they should be forced to take it. טוב אחרית דבר מראשיתו (Koheles 7,8).
With this we can understand the Medrash which says the angel of Yam Suf refused to split its waters until it saw the wealth that Am Yisrael took from Mitzrayim. The Yam refused to split it waters for then it would be changing the laws of nature which Hashem gave to it. However, when it saw the wealth of Mitzrayim in Jewish hands who reluctantly took it against their nature as the passuk says וישאילום they were forced to do so, then the Yam also decided to change its nature and split its waters. They were forced because Avraham programmed them with the ability not to take money if will ill affect them spiritually and here because Hashem asked them to do so they acted accordingly.

Rav Brazil
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