Some Tikkun Olam Sources in Mishnah

(2) At first, a man [who had already sent his wife a bill of divorce by means of a messenger] would set up a religious court in a different place [from where the wife lived] and cancel [the bill of divorce]. Rabban Gamliel the Elder enacted that they not be able to do this, due to [the need for] repairing the world [Tikkun HaOlam]. At first, a man could change his name and her name, the name of his city, or the name of her city. Rabban Gamliel the Elder enacted that one would write: "The man, So-and-so, and any other name that he has, and the woman, So-and-so, and any other name that she has," due to Tikkun HaOlam.

(3) ...Hillel instituted a document that prevents the Sabbatical Year from abrogating an outstanding debt [prosbol] for the betterment of the world, as the Gemara will explain.

(3) [Any loan made with a] Pruzbul [court exemption from the Sabbatical year cancellation of a loan] is not canceled. This is one of the matters that Hillel the elder instituted. When he observed that the nation withheld from lending to each other and were transgressing what is written in the Torah (Deuteronomy 15:9): "Beware lest there be in your mind a base thought," he instituted the Pruzbul.

(5) One who is half slave and half free-man, serves his master one day and [works for] himself one day. These are the words of Beit Hillel. Beit Shammai said to them: "You have repaired [the situation] for his master, but for himself you have not repaired it. To marry a maidservant is impossible [i.e., forbidden], for he is half-free. [To marry] a free-woman is impossible, for he is half-slave. And was not the world created for the sake of reproduction, as it says (Isaiah 45:18) "Not for emptiness did He create it, but for settlement He formed it." Rather, due to Tikkun HaOlam, we force his master and he makes him a free-man, and [the slave] writes a document [of debt] for half his value. Beit Hillel retracted and ruled in accordance with the words of Beit Shammai.