remittance bill; money order
A financial instrument used to send money, especially in banking or postal contexts.
I send money abroad using a remittance bill.
為替 is a broader term for money exchange or a bill of exchange, while 送金為替 specifically refers to a remittance bill for sending money.
送金 is the act of remitting money, whereas 送金為替 is the instrument or document used for that remittance.
Compound of 送金 (remittance) and 為替 (money exchange/bill). The exact historical derivation is uncertain; the spelling is conventionally associated with financial remittance instruments.