## Refund a payment

Creates a refund for a captured payment that has been billed.
Supports partial refunds by specifying an amount less than the
remaining refundable amount. Defaults to full refund if amount
is not specified.

## Endpoint signature

```http
POST https://api.reservepay.com/merchants/refund-payment HTTP/1.1
Content-Type: application/json
Accept: application/json
Authorization: Bearer <token>

{
  payment_id: string,
  amount: number?,
}
```

Returns: string

## Request arguments

### `payment_id` (string)

**Required**. The unique identifier of the payment to refund.

### `amount` (number)

_Optional_. The refund amount in the smallest currency unit (e.g. satang).
Defaults to the full refundable amount if not specified.

## Errors specific to this endpoint

### `NOT_FOUND`

No payment with the provided ID exists in your merchant account.

### `NOT_REFUNDABLE`

Payment is not eligible for refund. This can happen if the payment
has not been billed yet, is already fully refunded, or the integration
does not support refunds.

### `AMOUNT_EXCEEDS_LIMIT`

The refund amount exceeds the remaining refundable amount.

## Errors common to all endpoints

### `UNHANDLED_ERROR`

This error occurs when the server encounters an unexpected internal error that
it cannot handle gracefully. This typically happens due to bugs, infrastructure
issues, or edge cases that weren't anticipated during development.

### `INVALID_ARGUMENTS`

This error occurs when the request contains invalid or missing parameters.
Common cases include missing required fields, or values that don't match the
expected format or type.

### `BAD_VERSION`

This error occurs when making requests to an API version that does not exist.
This commonly happens when using an outdated SDK or when the API version
specified in the request URL is incorrect.