It is popularly known as debit memorandum (memo).
The seller may act upon this debit note to issue new invoice or debit note as an adjustment on amount of purchase issued in previous invoice.
Such transaction may attract credit as seller complete the shipment first and then receives the vary amount.
Generally, real sale is done when goods are exchanged for money while issuing invoices.
Debit note and credit note are both part of accounting system and are used to keep a track or to follow the trail for future transactions.
Apart from formal letter format debit notes may be issued as an shipping receipt that disclose information regarding goods received by the buyer and the amount due to the seller is duly noted as payment is not expected by the seller until and unless official invoice is generated and posted in favour of the buyer.