Creditor did not have a post-petition secured interest in debtor-hotel's receipts. By the terms of the mortgage, the creditor's secured interest in rents, profits, etc., did not vest until default and enforcement of the mortgage. The interest did not vest pre-petition because the creditor did not enforce the mortgage pre-petition. The interest could not be perfected post-petition under § 546(b) because no state law allowed the creditor's post-petition perfection to relate back to a time pre-petition so as to defeat the lien avoidance powers of the debtor-in-possession.