An invoice is a commercial document issued by a seller or a buyer, relating to a sale transaction and indicating the products, quantities and agreed prices for products or services the seller had provided the buyer according to wikipedia.
Payment terms are usually stated on the invoice. These may specify that buyer has a maximum number of days in which to pay, in the other words due date. The buyer could have already paid for the products or services listed on the invoice.
From the seller point of view, an invoice is a sales invoice. From the point of view of a buyer, an invoice is a purchase invoice. The document indicates the buyer and seller, but the term invoice indicates money is owed or owing.