In ADMIN>>>LOCALIZATION>>>CURRENCIES
You have to SET UP Singapore Dollars as a Currency.
I imagine that the " $ " currency you are using is the default setting, which usually applies to US $, and will have a code of USD in the " CODE " field of the currency description.
When you are in the currency admin screen, click "NEW CURRENCY" button.
Enter the following data:-
Title: Singapore Dollar
Code: SGD
Symbol Left: SGD$
Symbol Right:
Decimal Point: .
Thousands Point: ,
Decimal Places: 2
Value: 1.000000
YES: Set as default (requires a manual update of currency values)
NOTE: Set your Decimal Point, Thousands Point and Decimal Places, to suit your needs.