Bava Metzia 69: three things to remember

A partner may unilaterally divide identical coins because money is fungible, but cannot divide wine barrels whose quality varies unseen.
Renting out coins for a daily fee is forbidden usury, but paying a broker to persuade a lender is entirely permitted.
Increasing rent after loaning funds to improve a field, paint a shop, or add a sail is permitted investment, not interest.