Interest on loans

In Bava Metzia there is the היתר עיסקא. I forget the exact arrangement.From what I recall in Bava Metzia ch 8 it is you give  a fellow a certain amount of money to buy goods and to sell at a profit. Half is a loan that must be paid back.The other half is a partnership. if a loss is incurred both share the loss. If profit then both share the profit.