POST /v2/transactions — fx-spot-transaction

A spot FX trade between two cash accounts in different currencies.
Inflow side must be positive, outflow side must be negative; each
amount is denominated in the currency of its own account.

Request:
{
  "data": {
    "type": "fx-spot-transaction",
    "attributes": {
      "inflowAccountAmount":  "number (required) — positive, in inflow-account currency",
      "outflowAccountAmount": "number (required) — negative, in outflow-account currency",
      "comment":              "string (optional)",
      "tradeDate":            "string, ISO 8601 (required) — trade date",
      "valueDate":            "string, ISO 8601 (required) — value date"
    },
    "relationships": {
      "inflowAccount":  { "data": { "type": "account", "id": "string (required)" } },
      "outflowAccount": { "data": { "type": "account", "id": "string (required)" } }
    }
  }
}

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "fx-spot-transaction",
    "attributes": { /* same as request */ },
    "relationships": { /* same as request */ }
  }
}