Legal Fiction, Torah and Life

The commands of G-d in the Torah are eternal. We are commanded not to add or detract from them. However, the rabbis are authorized by the Torah (Deut. 17-8-13) to interpret, and do whatever needs to be done to enforce and protect the laws of the Torah. But what happens if society changes, and a law that was intended to help people, now hurts them?
A classical example of this is one aspect of the Sabbatical Year (Shmitah). Aside from the agricultural rules that apply, there is the seemingly strange rule of the cancellation of debts. In an earlier time, this had the effect of eliminating a permanent debtor class from among the people of Israel.In many countries in Europe, until two centuries ago, debtors were put in prison, or worse, sent to live in colonies in distant lands. In the Torah, a small farmer who owed a few sheqels would be free of his debt when the shmitah year came and went. But what about in later times? Being able to borrow large sums of money is a major feature of conducting a large business venture. If a businessman would go to a banker to borrow a million dollars, and in a few months is shmitah, at which point the debt would be cancelled, this would probably put a dent in the banker's enthusiasm to make the loan! Although the Torah warns NOT to have that attitude, that is well and good if my neighbor wants to borrow five dollars, and may not be able to pay me back. But what if he's borrowing five million dollars? Shmitah will be in a short time., He fully INTENDS to pay me back. But is it reasonable for me  to take such a risk?
The great sage, Hillel, saw the dilemma. The Torah wants to protect the borrower, but now, because of the law in the Torah, the borrower now finds himself unable to raise the capital needed for his business! Hillel devised a ha'aramah (legal fiction) called the prosbol. According to the law, only private debts are cancelled. A debt owed to the community remains intact. Hillel instituted that before the moment that the debts are cancelled  (actually the final moments of shmitah year), an individual who is owed money could present his contracts to a court (Beit Din), thus making the debts public. The court would appoint the creditor as its agent to collect the debt. Now the person needing the money will be able to find it, without the lender refusing to give the money. Life, as it had become in a mercantile world, could go on.
But is this really acceptable? Can the Torah be circumvented this way? The Talmud brings a very different example. Produce of the Land of Israel must be tithed That is, a portion must be given to the Levites and Kohanim (priests) "of whatever passes through your gates". People were getting around this by lowering the produce into their homes through chimneys! The rabbis cursed whomever did this, and made a rabbinic ruling that all produce brought into one's home was bound by the obligation of tithing.
How is this different from Hillel's prosbol? In the case of the produce, an "out" was sought to enable an evasion of the law if the Torah. In the case of the prosbol, the original intent of the law, the protection of one seeking to borrow money, was upheld.
Torah is life! G-d gave the Torah over to the sages and judges to make sure that life keeps pace with the Torah,  and the Torah with life!