No parish treasurer shall be allowed credit for any voucher he presents in the settlement of any account, until he has first taken an oath that he has paid the full amount of the voucher in money, or that he has received the same in payment of parish dues, and that he has not speculated thereon himself, nor has he been directly or indirectly interested in any speculation that may have been made in its acquisition.
La. R.S. § 33:1660