From Wikipedia, on accounting:
Double-entry bookkeeping was pioneered in the Jewish community of the early-medieval Middle East and was further refined in medieval Europe. With the development of joint-stock companies, accounting split into financial accounting and management accounting.
The first published work on a double-entry bookkeeping system was the Summa de arithmetica, published in Italy in 1494 by Luca Pacioli (the "Father of Accounting").
And about Descartes:
One of Descartes's most enduring legacies was his development of Cartesian or analytic geometry, which uses algebra to describe geometry. Descartes "invented the convention of representing unknowns in equations by x, y, and z, and knowns by a, b, and c". He also "pioneered the standard notation" that uses superscripts to show the powers or exponents; for example, the 2 used in x2 to indicate x squared.
We often take these things for granted.
I have been in the position of someone transported to a place where people don't know basic math often enough that I can sympathize with any time travelers going to the middle ages. In my case, it was more like being an IT pro with more than one boss who got their position through the Dilbert principle. Seriously. I once made a piece of software that would keep some people from using some company accounts if they went overbudget for the month. I had to explain this to my boss multiple times, and he still wouldn't have it.
"How come we are over budget if there is still money in the account?"
"Well, there are some expenses that the company has to pay for in a few days. You use that money now, you won't be able to pay everyone's salaries and the electricity bill"
"I still don't get it, this system sucks! Turn it off!"
There's this book I really love, called "The Richest Man of Babylon". It tells the story of a filthy rich merchant in ancient Babylon who is tasked by the king with teaching basic math to his subjects so that the realm can generate more wealth. In one of the lessons, rich guy tells everyone that they must have a budget. A young guy gets mad at the notion that he must be a slave to a table and ragequits the class. He ends up poor while most others end up rich.
You've seen it too. Every one has at least one friend or relative who sucks at budgets, and is always either wasting money on superfluous things or getting into debt without actually having to. In order to stop sucking at budgets, the very least you have to do is a table with incomes and expenses, which is a rudimentary way of bookkeeping. People who suck at math or can't read and write will have a very hard time at this. In the middle ages most people would fit into one group or the other, sometimes both.
So if you are able to make good choices with money, you could keep your overlord from going bankrupt. If you're really smart and clever, or if you graduated in a STEM course you can probably come up with a system of your own that will improve the local economy a little bit. But if you are an actual accountant? You might be interested in reading about the Medici family in Italy. They opened a bank that lasted from 1397 to 1494 and made them the wealthiest family in the western world until incompetence caused the bank to be liquidated. You could probably replicate the feat in earlier dates, and even make it more durable since you'd have further financial and historical knowledge.