Banking and Money
How does this sort of business get recorded?
The real question is: after a year of such business, did we make any money?
There's a real drive because many business partnerships last only a few years.
A chronological journal of all transactions: borrowing, buying, selling, all mixed in a single book. You would have to go back through and do a running total in order to figure out how much you made.
Worse, various transactions (e.g., loans to a family member) may not have been recorded. The result was that merchants often didn't know if they were profitable, they only knew if there was money in the strongbox. This is the way lots of nobles and even princes ran their treasuries, too.
Italians developed this. We have a 15thc book on the technique, but the practice is surely older than that.
Credits and debits kept in columns. All you did was add them up. The loan and the repayment were on the same line, just recorded at different dates.