Economics
In the city-state of Ur, or anywhere in Mesopotamia, there was no actual paper money or coins. Everything was done by bartering. Merchants traded their agricultural surpluses with each other and with people from other civilizations.
The ruling elites also collected taxes on agricultural surpluses. The cuneiform writing system's original purpose was to document and record who owed how much in 'tax money'. People could also pay off theur debts by participating in community projects. These projects included building structures for society, such as ziggurats. People with massive debt had the option to become slaves. By giving up their freedom, their debt was lifted.
On a side note, agricultural surpluese were given to priests in exchange for them intervening with the gods. By intervening, they were believed to have brought good fortune for the people.