It depends on if it is a non-refundable deposit, retainer, etc.
The remaining $1,500 is not included in that quarter's sales, because you have not yet received it and it is not guaranteed.
The question is really if you should count the $500 toward the quarter where it is received, or during the quarter where you invoice. This deposit might be categorized as a liability until you invoice, and there is no sales tax to be calculated until the invoice for the total.
I say 'might' because this can vary by state and the type of transaction or business. For example, if someone makes a cash down payment on a lease for a car, some states will require that sales tax be charged on this.