curl -X POST 'https://api.goatpay.com.br/v1/billings/refund/clx_billing' \
  -H 'X-API-Key: gp_live_SUA_CHAVE' \
  -H 'Content-Type: application/json' \
  -d '{ "amount": 50 }'

{
  "success": true,
  "message": "Cobrança estornada",
  "data": {
    "id": "clx_billing",
    "status": "REFUNDED",
    "statusLabel": "Estornado",
    "billingType": "PIX",
    "amount": 79.9,
    "netAmount": 76.91,
    "feeAmount": 2.99,
    "notificationFeeAmount": 0,
    "installmentCount": 1,
    "description": "Pedido 123",
    "dueAt": "2026-06-15T23:59:59.000Z",
    "dueDate": "2026-06-15",
    "paidAt": "2026-05-31T14:22:00.000Z",
    "estimatedCreditDate": "2026-06-02T00:00:00.000Z",
    "invoiceUrl": "https://...",
    "bankSlipUrl": null,
    "digitableLine": null,
    "pixCopyPaste": "00020101021226730014br.gov.bcb.pix...",
    "pixEncodedImage": "iVBORw0KGgo...",
    "accountTransactionId": "clx_ledger",
    "externalReference": "pedido-123",
    "createdAt": "2026-05-30T10:00:00.000Z",
    "customer": {
      "id": "clx_customer",
      "name": "Cliente Exemplo",
      "taxId": "12345678909"
    }
  },
  "requestId": "req_abc123"
}