Could you explain how shmittah is an ethical teaching? Just seems like good farming practice. Also, returning lands to their owners may be nice for the seller, but not necessarily for the buyer.

The buyer should take it into consideration when paying the price, so no damage done to the buyer.

It is ethical teaching. First of all, there are the debt-forgiveness and other things that aren't about farming at all. Second, saying everyone has rights to the produce during Shemittah is a major change from the norm (both 6 years out of 7 and everywhere else) of "what's mine is mine". And finally, saying Shemittah is just about good farming practices is like saying "don't eat pork" is about trichinosis.

Debts are cancelled after shmitta but land sales are only cancelled at yovel.

@manasseh none of the things you said are ethical teachings.

@AA Shameful mistake by me, edited. Also, there is ethical teaching when God tells us that our ownership on our land is limited, and people should get a fresh start.

@manassehkatz my point wasn't the reason behind the mitzvah. My point is it's not unique to Judaism, as many farmers practice letting the land lie fallow

@Alaychem slaves also go free at yovel not shmitta. If the ethical teaching is that removed from the rule, it's also not unique to judaism. Many if not all world cultures teach about second chances.

@AA It seems that we understand the question differently. Since the Tora lean towards practical actions, rather then abstract principles, I thought that the OP meant "which practical actions with ethical meaning behind them are still unique to the oral Tora". If the OP meant like your understanding, my answer is indeed incorrect.

@AA "slaves also go free at yovel not shmitta" - No. Shmot 21:2

That verse is not talking about shmitta...

Slaves going free is from Leviticus 25 explicitly about Yovel וְכִי-יָמוּךְ אָחִיךָ עִמָּךְ, וְנִמְכַּר-לָךְ--לֹא-תַעֲבֹד בּוֹ, עֲבֹדַת עָבֶד. כְּשָׂכִיר כְּתוֹשָׁב, יִהְיֶה עִמָּךְ; עַד-שְׁנַת הַיֹּבֵל, יַעֲבֹד עִמָּךְ. וְיָצָא, מֵעִמָּךְ--הוּא, וּבָנָיו עִמּוֹ; וְשָׁב, אֶל-מִשְׁפַּחְתּוֹ, וְאֶל-אֲחֻזַּת אֲבֹתָיו, יָשׁוּב

Rambam Shmita veYovel 10:17 יתרה שביעית על היובל--שהשביעית משמטת כספים, ואין היובל משמיט כספים. ויותר יובל על השביעית--שהיובל מוציא עבדים, ומשמיט קרקע

@AA You are correct. Fixed