Every contract, purchase order, or other document purporting to create a financial obligation against the City shall, in order to be valid and enforceable, bear a certificate signed by the Director of Finance stating that funds are available from which to pay it. Such certificates shall be affixed before the document is delivered by the City to the vendor or contractor and before any goods are delivered or services rendered. At the same time, the estimated amount of the obligation created thereby shall be entered as an encumbrance against allotted funds, which shall thereupon not be available to finance any other transaction, to the entent of such encumbrance.