Segregation of Duties (SoD)
The control that requester, approver and recorder must be three different people. Collapse any two of those roles into one pair of hands and you have built the fraud scheme yourself, in your own policy.
The classic construction application is asset disposal: the person who sells the asset must not be the person who records the sale, and the proceeds must be handled by someone other than the seller. Break that and you have created the scheme — sold it, pocketed it, marked it scrapped.
And it has to be enforced structurally, by configuration, rather than by vigilance. Vigilance fails on a Friday afternoon when the approver is on leave and somebody is being helpful. Configuration does not, and that is the whole argument for putting it in the system.