POST /v2/transactions — refund-transaction

Note: A refund transaction will affect the account balance, but the
return is reflected on the corresponding refund instrument — not on
the original investment instrument.

Request:
{
  "data": {
    "type": "refund-transaction",
    "attributes": {
      "accountAmount": "number (required) — amount deposited/withdrawn, in account currency",
      "comment":       "string (optional)",
      "tradeDate":     "string, ISO 8601 (required) — trade date, end-of-day",
      "valueDate":     "string, ISO 8601 (required) — value date, end-of-day"
    },
    "relationships": {
      "account": { "data": { "type": "account", "id": "string (required)" } }
    }
  }
}

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