Is the $30 less a discount for early payment or is the customer going to pay that $30 in the future?
In the Receive Payments window, you apply the $2000 against that particular open Invoice. It will leave $30 still due on your Accounts Receivable. If this is a discount, select the Discounts & Credits button and apply the $30 to a Sales Discount income account. You may have to create one.