In Gitten 36a it brings Rebbi's Shita that SHvius nowadays are only MiDarabanan (as Paraphrased by Kollel Iyun Hadaf)

a) (Mishnah): Hillel enacted Pruzbul...

(b) (Mishnah): A loan for which a Pruzbul was written is not cancelled is Shemitah - this is one of Hillel's enactments.

(c) Question: Do we ever find, mid'Oraisa the loan is cancelled, and Hillel enacts that it should not be cancelled?!

(d) Answer #1 (Abaye): The Mishnah is like Rebbi, who says that cancellation of loans in Shemitah is only mid'Rabanan nowadays.

1. (Beraisa - Rebbi): "This is the matter of Shemitah - Shmot (withhold)".

i. The verse speaks of 2 withholdings - not working the land, and not collecting loans.

ii. When we may not work the land, we may not collect loans; at a time when the law of not working the land in Shemitah does not apply, neither does the law of canceling loans.

36b---------------------------------------36b

iii. Chachamim enacted cancellation of loans in Shemitah (nowadays) as a remembrance of Shemitah; Hillel saw that people were not lending, so he enacted Pruzbul.
In 36b Tosfos D"H V'Tikun asks why they enacted Shvius and not Yovel

Tosfos answers that most of the nation would not be able to keep such an enactment to refrain from working the land for 2 years in a row, so they don't enact in such circumstance.

The Sma asks what's so hard, the Torah promises that there would be enough grain for all those years?

He answers that is only when he keeps the Torah's precept of Shvius, but here it's only rabinic obligation which the Torah doesn't promise about it.

The Chazon Ish argues that if the Rabanan enacted it, then they did it with the same promise, like the Torah precept.

Rather, we must worry that one's sins might make that the promise to become null as we see in Brachos 4a

David told Hashem, "I know that You pay a good reward to Tzadikim in the World to Come, but I do not know whether or not I will have a share among them, for perhaps I will lose my portion on accountof sin!"

i. Contradiction (R. Yakov bar Idi): Hashem promised Yakov, "... u'Shemarticha b'Chol Asher Telech," yet it says, "va'Yira Yakov Me'od"!

ii. Answer: He was concerned that he sinned and thereby forfeited Hashem's promise.

i. (Beraisa): "Ad Ya'avor Amcha Hashem Ad Ya'avor Am Zu Kanisa" -- these refer to the two timesYisrael entered Eretz Yisrael, in the days of Yehoshua and the days of Ezra;

ii. The verse equates them -- Yisrael should have entered the second time amidst miracles like the first time; they did not, because they sinned.