The original poster is asking what gives fiat its initial value.

In the United States, the market set the initial value of currency. Originally, no force was used on the American people. They used British pounds and other currencies even after the revolution. Americans were not "forced" to use dollars. See A Midwife's Tale, a diary by Martha Ballard (1734-1812) who details among other things, every trade and purchase she made in the last 27 years of her life.

Other countries may have used force to initially impose currencies, but not in the United States.