Cash memo vs. invoice vs. receipt
A cash memo is essentially an invoice and receipt combined into one document, used when payment is made instantly at the point of sale. An invoice is issued when payment is expected later; a receipt is issued after payment has been received. A cash memo collapses both steps because the money changes hands immediately.
This makes cash memos the natural document for retail counters, restaurants and small workshops. There is no need for a two-step invoice-then-receipt workflow when the customer is standing in front of you paying cash. The memo itself is the entire paper trail for that sale.
For higher-value sales, credit sales, or B2B transactions with payment terms, you should switch to a proper invoice and issue a separate receipt when payment arrives. Mixing cash memos with credit sales creates bookkeeping confusion and weakens your audit trail.