curl --request POST \
  --url https://sandbox-api.billingrails.com/v1/credit_grants/{id}/apply \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "invoice_id": "<string>",
  "reference": "<string>",
  "used_amount": 123,
  "description": "<string>"
}'

{
  "credit_grant": {
    "object": "credit_grant",
    "id": "cgr_IhMUl3rrZ3",
    "created_at": "2025-02-14T17:33:40.843Z",
    "account_id": "acc_IhMUl3rrZ3",
    "type": "promotional",
    "status": "granted",
    "payment_status": "comped",
    "currency": "NGN",
    "grant_amount": 5000,
    "available_amount": 3000,
    "expires_at": "2025-03-14T17:33:40.843Z",
    "effective_at": "2025-02-14T17:33:40.843Z"
  }
}