$ appears to have evolved ca 1775 in the United States from a common abbreviation for pesos, also known as piastres or pieces of eight, a P/raised-S ligature PS that passed through a stage resembling ֆ. It was used in the US before the adoption of the dollar in 1785.
When used as a currency symbol, $ precedes the number it qualifies (in English), despite being pronounced second. For example, “$1” is read as “one dollar”, not “dollar one” unlike the usage in languages such as French or German: “1 $”, “2,50 $”.
When used for the Portuguese escudo, $ is placed between the escudos & centavos, 2$50. The official symbol for the escudo is (with two bars), but because of the continued lack of support in Unicode, a single bar dollar sign is frequently employed in its place even for official purposes.