Must a man return the amount of money he borrowed, or the quantity of gold that was sold for his sake, especially since the price of gold has risen and this was not agreed upon beforehand?

The debtor is not obligated to pay you more than the ten thousand (currency unit) they borrowed. The principle for repaying debts is based on the likeness (of the loaned amount), not on its value. It is not permissible to link fixed debts (obligations) to the price level. The debtor should, however, perform well (in repayment) and return the favor.