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From the wonderful website, emishnah.com:

(3) [A loan secured by] a prosbul [the next Mishnah explains the term] is not cancelled [by shemittah. This was one of the things instituted by Hillel HaZakan; for when he saw that people were refraining from lending to one another, and were transgressing that which is written in the Torah, “Guard yourself, lest there be in your heart an unfaithful thought, saying, The seventh year, the year of the release has approached and you will be miserly with your needy brother and you will not give him; (Deuteronomy 15:9).”] Hillel then instituted the prosbul. [It should be noted that, at the time of Hillel, since the laws regarding the return of properties during Yoveil were no longer in force, the laws regarding the cancellation of loans of shemittah were also not in effect, as according to this opinion (see Gittin 36b), one was dependent on the other. However, the Rabbis reinstituted the laws of shemittah, so Hillel was not circumventing Biblical law].

And, as those [bracketed] notes make clear, it is an established and interesting question just how Hillel could have done this, cancelling a mitzvah in the Torah. Is this proof that the Torah is modifiable by the Rabbis?

The answer above is that Hillel was only cancelling something which was of Rabbinic, rather than Biblical status.

I would answer that, in fact, Hillel was making a derasha. The pasuk was {Devarim 15}:

Hillel interpreted pasuk 9 to be a positive commandment, rather than a negative commandment. הִשָּׁמֶר לְךָ, that is, take positive steps to guard against something. Specifically, פֶּן-יִהְיֶה דָבָר עִם-לְבָבְךָ בְלִיַּעַל לֵאמֹר. This is not a guard against action. It is a guard against a particular thought and intention. While the Torah does not outlaw thought but just action, the Torah can positively command one to take action to prevent a thought. Finally, this positive command was one fulfilled by Chazal in general and Hillel in particular. And the action was establishing the mechanisms of prozbol.

This was a positive mitzvah which did not always hold true. It depended on the metzius. For years, people would indeed still regularly lend. But, when Hillel saw that this devar belial was prevalent, then this other positive mitzvah kicked in.

And so, the law to cancel loans at shemitta is Biblical. And the law to prevent the cancellation of loans (in a particular circumstance) is Biblical, and they complement one another.

Further, how was this positive mitzvah accomplished? There may well be mechanics by which loans are already exempt from cancellation at shemitta. Thus, the pasuk stated:

and the derasha is specifically your brother’s hand, and not that which is already in your hand (that is, if you had collateral in hand) or that which is in bet din‘s hand (by handing his shtar to the bet din). Or, it might be via hefker bet din hefker, whereby the loans are indeed cancelled but then immediately restored to validity. If so, this is not nullification but employment of a clever loophole. And add to this loophole, the above, that this was a commanded loophole.

Related in approach, perhaps, is the Netziv:

So how could Hillel come up with this circumvention?

The answer, in fact, is in the aforementioned text. According to the Netziv, Rabbi Naftali Tzvi Yehudah Berlin, in his classic commentary Ha’amaik Davar, we must focus on verse 15:4. Directly after telling us about the law of remission, the Torah says, “Nevertheless, there shall be no needy among you.” Can this mean that there will be no needy people? Clearly, history teaches otherwise. Further, a few verses later, the Torah itself states, “For there will never cease to be needy within the land.” (15:11). What, then, can the statement “Nevertheless, there shall be no needy among you” mean?

Says the Netziv, “The Torah itself is stating that this law [of debt remission] should not be the cause of poverty. If commercial lenders will not do business, as their capital will not be returned, the credit markets will disappear and small business owners, other needy borrowers, and some lenders will in fact fall into poverty. The Torah, then, is explicitly saying to keep the law of remission insofar as it prevents poverty, but not if it causes it.”

Hillel, far from circumventing the Torah, was following it precisely.