Materials, supplies, equipment, and services may be procured by open market purchase or sole source award when the amount involved is anticipated to be less than $2,500 in the aggregate on an annual basis for a particular procurement. Procurement by open market purchase or sole source award in amounts over $2,500, but less than $10,000, is permitted when the materials, supplies, equipment, and services are of a unique and special nature, or when public exigency will not permit the delay incident to competition. In this event, a memorandum justifying the action and signed by the Executive Director shall be filed with the official records of the Authority.