No agreement entered into by the board of supervisors for the purchase, hire, or rental of any apparatus used in the construction, improvement, or maintenance of highways shall create a charge against the county, unless the agreement complies with all of the following:
(a)
The agreement is in writing.
(b)
The writing is signed by the chair of the board of supervisors.
(c)
A copy of the writing is certified by and filed with the clerk of the board of supervisors.
All of these writings and copies are public documents.
If a county purchases, hires, or rents any apparatus specified herein pursuant to a local purchasing ordinance, including competitive bidding procedures, it does not have to comply with subdivisions (a), (b), and (c) herein to create a charge against the county. Compliance with the local purchasing ordinance shall be sufficient to create a charge against the county for said purchase, hire, or rental.