PayPal\Api\Refund::setInvoiceNumber PHP Method

Your own invoice or tracking ID number. Character length and limitations: 127 single-byte alphanumeric characters.

public setInvoiceNumber ( string $invoice_number )
$invoice_number	string

    public function setInvoiceNumber($invoice_number)
    {
        $this->invoice_number = $invoice_number;
        return $this;
    }

Refund