I'm convinced you're not even bothering to read my posts which are typically less than 5 sentences. I made no assumptions on how much he started with, cuz it does not matter.
Here, I'll walk you through the example of him starting with $0 and using credit cards to make the purchase. First he buys a goat for $60, so he owes Amex $60. Then he sells that goat for $70 cash. Then he buys that goat again for $80, so now he owes Amex $60+$80=$140. But then he sells the goat for $90 cash, so he had a total of $70+$90=$160 cash. He then takes that cash and pays Amex the $140 he owes them and he's left with $160-$140=$20. That's his profit. That's how much money he made. It's not any more complicated than that.
You could repeat this exercise with him starting with
any amount of money and as long as there is no cost to borrowing money, the profit will always be $20.