Initiates a new payout from the authenticated merchant to a payee via a specified instrument.
Merchant API key: ak_live_* (production) or ak_test_* (sandbox).
Unique key to ensure idempotent request processing.
"pye_abc123"
"ins_abc123"
"1000.00"
"850.00"
"EUR"
"bank_transfer"
255100"USD"
fiat, crypto, stablecoin merchant, payee fixed_source, fixed_dest Payout created.