When a judgment creditor has caused the debtor to be committed on execution and afterwards discovers goods, effects or credits of the debtor not attachable by ordinary process of law, he may have the benefit of the trustee process like any other creditor if, within 7 days after service of the process, he discharges the debtor from prison by a written direction to the jailer stating the reason therefor, but such discharge shall not annul or affect the judgment.