In order to comply with G.S. § 159-26(d), which require encumbrance accounting systems in cities with a population over 10,000, each purchase shall be evidenced by a purchase order. The purchase order shall be issued by the Purchasing Agent or his delegate at the time the purchase negotiations are complete. The purchase order may consist of any suitable format recommended by the Purchasing Agent, so long as that format provides ample information to allow proper encumbrance of anticipated expenditures against budgeted appropriations. The Purchasing Agency is hereby granted the authority to establish a reasonable de minimis amount under which no purchase orders are required.