שמיטת כספים and פרוסבול

In last week's parsha, Behar, the Torah writes the din of שמיטת כספים. The gemara in Gittin 36a states:
הלל התקין פרוסבול וכו': תנן התם פרוסבול אינו משמט זה אחד מן הדברים שהתקין הלל הזקן שראה את העם שנמנעו מלהלוות זה את זה ועברו על מה שכתוב בתורה (דברים טו) השמר לך פן יהיה דבר עם לבבך בליעל וגו' עמד והתקין פרוסבול

Hillel saw that people were not lending money and therefore created פרוסבול so that the loans would not be canceled by shemitta.

Many misguided people look at פרוסבול and say that from here we see that the Rabbis can do whatever they want, where there is a rabbinic will there is a rabbinic way. In fact, as I will show פרוסבול shows us just the opposite.

The gemara asks on Hillel
ומי איכא מידי דמדאורייתא משמטא שביעית והתקין הלל דלא משמטא

How could Hillel be מתקן פרוסבול when the torah says that the loan is canceled?
The gemara answers:
אמר אביי בשביעית בזמן הזה ורבי היא

Abaye answers that פרוסבול only works if shemitta is d'rabbanan. The gemara then asks the reverse question:
ומי איכא מידי דמדאורייתא לא משמטא שביעית ותקינו רבנן דתשמט

How could the chachamim make shemitta derabbanan, min hatorah he has to pay back the loan? The gemara answers
רבא אמר הפקר ב"ד היה הפקר

Beis Din has the power min hatorah to take away your money.

The simple reading of the gemara is that פרוסבול only works if shemitta is d'rabbanan. In other words, the Rabbis don't have the power to what they want, if shemitta is min hatorah they can't do anything. In fact, this is how the Rambam (הלכות שמיטה ויובל פרק ט) paskens ואין הפרוזבול מועיל אלא בשמיטת כספים בזמן הזה, שהיא מדברי סופרים; אבל שמיטה של תורה, אין הפרוזבול מועיל בה

The Raavad there argues on the Rambam and has a different interpretation of the gemara, Rashi also learns like the Raavad.

They explain the gemara as follows. Rava's answer of הפקר ב"ד היה הפקר answers the original question as well. What is the machlokes the Rambam and the Raavad? Here are 2 possible explanations:
1. There is a famous machlokes what is the power of הפקר ב"ד היה הפקר min hatorah? Is it a) the simple translation of the words that beis din can declare your property ownerless or is it more then that, b) they can take your property and give it to someone else.
One case where this comes up is where a man is mekadesh a woman with a kinyan d'rabban, is she married min hatorah? Kinyanim d'rabban work based on הפקר ב"ד היה הפקר if we hold like a, then a kinyan d'rabbana only works m'drabbanan, min hatorah the woman has not yet received the money and therefore min hatorah is not yet married. However, according to b, a kinyan d'rabbana works min hatorah and she is married min hatorah.

Based on this we can understand the machlokes the Rambam and the Raavad. The Raavad holds like b, that Beis Din can take from a and give to b, that is how a פרוזבול works, Beis Din takes the money from the borrower and gives it to the lender before shemitta, therefore there is no loan for shemitta to cancel. the Rambam on the other hand holds like a, Beis Din min hatorah can only take away your money but they cannot give it to me and therefore it doesn't help for shemitta, by shemitta they need to give you the money and they can't, and therefore shemitta cancels the loan.

The gemara there brings 2 sources from where הפקר ב"ד היה הפקר is learned out. דאמר ר' יצחק מנין שהפקר ב"ד היה הפקר שנאמ' (עזרא י) וכל אשר לא יבוא לשלשת הימים כעצת השרים והזקנים יחרם כל רכושו והוא יבדל מקהל הגולה רבי אליעזר אמר מהכא (יהושוע יט) אלה הנחלות
. The Rashba seems to say that the above machlokes depends on what the source is. The pasuk in Ezra is like a, Beis Din can take away your money, while the pasuk by nachala is like b (they took from 1 and gave to another). The Rambam when he brings down the din of הפקר ב"ד היה הפקר quotes the pasuk in Ezra as the source, the Rambam lshitaso that הפקר ב"ד היה הפקר is like a, and doesn;t work for shemitta min hatorah.

2. How does shemitta cancel a loan? a) Does it cancel the monetary aspects of the loan or b) does it NOT affect the monetary aspects, rather it prohibits the lender from collecting. The Rambam holds like b, it is an issur, therefore הפקר ב"ד היה הפקר is not relevant it can't be matir issurim, the Raavad on the other hand holds like a, and therefore הפקר ב"ד היה הפקר can restore the monetary aspects.

The gemara later om daf 37 has the following strange din
המחזיר חוב לחבירו בשביעית צריך שיאמר לו משמט אני ואם אמר לו אע"פ כן יקבל הימנו שנאמר (דברים טו) וזה דבר השמטה אמר רבה ותלי לי' עד דאמר הכי

If the borrower wants to return the money after shemitta anyway he can, but the lender must first refuse. Raba says that ותלי לי the borrower can do this to the lender until he says that he wants to pay back. What does ותלי לי mean? Rashi explains the gemara literally, the lender can string the borrower up on a tree and force him. The Rosh asks how could that be? This destroys the whole din of shmitta. We can explain the machlokes like 2 above. Rashi holds that shemitta doesn't cancel the loan, the borrower is still obligated, however, the lender cannot go and collect the loan. Therefore, he can force the borrower to pay as the gemara says. The Rosh holds that the loan is cancelled and therefore how can he force the borrower?

To conclude, we see that פרוסבול is an example of chachamim working within the halachic system and not just waving their magic wand. In the area of money the chachamim have more powerful tools to work with and may be able to do more, but in the end, they need to work with the halachic tools available.