Bava Metzia 67: The Dust of Interest
A case where a woman instructs a man to go buy property from her relatives on her behalf, which gives rise to the relative's question whether he'll be able to buy the property back, if and when he gets the money to do so. Which in turn gives rise to a discussion of whether that re-purchase can happen, and then the fruits of the field in the interim is either fully fixed interest or possibly a "hint" or the "dust" of interest (which is a rabbinic prohibition). Plus, how does the sale compare to a loan under comparable terms? Also, delving into a case of fixed interest and, over time, a reduction of debt. Plus, loans canceled due to the shemittah year.