What is the source and parameters for "going beyond the letter of the law"?

What constitutes actually giving up hope that a person will never recover their lost item

Using somebody else's item without their permission- is that stealing or borrowing?

How does the mechanism of yeush work on a legal and spiritual level?

When the roof of Rav Huna's Beit Midrash cracked and splintered

The difference between identifying a lost item using a siman vs. teviyut ayin

How to tell who the true signed name on a document is when there is more than one person with the same name in that town

Can a loan document that's already been paid back be used a second time for a loan?

How was Yaakov allowed to purchase the Birthright from Eisav if it was a "davar shelo bah l'olam"?

If someone gives someone a ride to their destination and at the end says pay me for the gas, are they required to pay given that the person was going anyway?