Is there a way to format a decimal as currency such as $220.00 USD?
No, this is not currently possible using a decimal field type, we do not have a "currency" appearance.
However, you could achieve this by using an additional text field (and set it to null esri field type if you do not want to add to feature layer) and then apply a calculation to display the value entered into the decimal field, and concat some extra text such as $ and USD around the value. This would get a display of the value you are after with the actual decimal value still being stored in feature layer.
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