def refund_amount(amount_in_cents = nil, refund_method = 'credit_first', options = {})
      return false unless link? :refund
      self.class.from_response(
        follow_link :refund, :body => refund_amount_to_xml(amount_in_cents, refund_method, options)
      )
    end
def refund_amount(amount_in_cents = nil, refund_method = 'credit_first', options = {})
      return false unless link? :refund
      self.class.from_response(
        follow_link :refund, :body => refund_amount_to_xml(amount_in_cents, refund_method, options)
      )
    end
Refunds the invoice for a specific amount.

 @return [Invoice, false] Invoice if successful, false if the invoice isn't
 refundable.
 @raise [Error] If the refund fails.
 @param amount_in_cents [Integer, nil] The amount (in cents) to refund.
 @param refund_method ["credit_first", "transaction_first", "all_transaction", "all_credit"] The method used to refund.
 @param external_refund [true, false] Designates that the refund transactions created are manual.
 @param credit_customer_notes [String] Adds notes to refund credit invoice.
 @param payment_method [String] Creates the manual transactions with this payment method. Allowed if *external_refund* is true.
 @param description [String] Sets this value as the *transaction_note* on the manual transactions created. Allowed if *external_refund* is true.
 @param refunded_at [DateTime] Sets this value as the *collected_at* on the manual transactions created. Allowed if *external_refund* is true.