משנה על גיטין 4:1

Whoever is half a slave and half a free man should work one day for his master and one day for himself, according to Beth Hillel. Beth Shammai said to them: “You have set matters in order with regards to his master, but you have not set matters in order with regards to himself. He is not able to marry a slave-woman, nor is he able [to marry] a woman who is free. Is he to refrain [from marrying]? [How can he] for is it not the case that the world was created in order for people to be fruitful and multiply? For it is said, “He did not create it to be a waste; but formed it for inhabitation” (Isaiah 45:18). But for the rightful ordering of the world his master is compelled to make him free, and he writes out a bond for half his value.” Then Beth Hillel changed their mind and taught according to the opinion of Beth Shammai.

[A loan secured by] a prozbul is not cancelled. This was one of the things enacted by Hillel the elder; for when he observed people refraining from lending to one another, and thus transgressing what is written in the Torah, “Beware, lest you harbor the base thought, [‘The seventh year, the year of remission, is approaching,’ so that you are mean to your needy kinsman and give him nothing.” Hillel enacted the prozbul.