Technical reference
Replace-by-fee: an unconfirmed payment is not final settlement
Mempool replacement policy and blockchain consensus are different things.
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Why replacement exists
BIP-125 describes opt-in replace-by-fee signalling. A sender can signal that a transaction may be replaced in a node's mempool, subject to the node's replacement policy. This supports fee adjustment when an unconfirmed transaction waits longer than expected.
The recipient's mistake
A wallet can display a payment before a block includes it. That display is evidence of a proposed transaction, not irreversible receipt. A replacement can spend overlapping inputs and change outputs. Conversely, lack of an opt-in signal should not be treated as a universal promise that no node or miner will accept a conflicting transaction.
Policy is not a protocol guarantee
BIP-125 records a particular replacement policy, not an immutable statement about every current node's configuration. Merchants should distinguish seen, confirmed and sufficiently confirmed states in their own records. A screenshot reading 'sent' does not establish which state the receiving business has actually verified.