Financial Management and HR Involvement
This incident should serve as a warning to all businesses that have purchasing functions. It is not strictly an HR matter, as these issues relate to the financial management of a company or business. Such discussions would be more appropriate on CiteFin. HR only gets involved when a mistake results in some form of "punishment" for an employee.
As Taj has pointed out, every organization should have a system of checks and balances in place regarding spending the company's money. This goes without saying. If you are aware that your organization lacks the necessary spending approval flow, then as an HR person responsible for the welfare of your employees, you should strongly recommend to management that they implement this immediately.
Spending Approval and Purchasing Guidelines
Simply put, discretionary spending by employees should be at a very low level, only for emergency purchases. Spending approvals for higher amounts should reside with senior management. Other than basic items like stationery and tea room supplies, no purchasing should occur without at least two written quotes from potential suppliers. All major purchases should be put out to competitive tender. All purchasing should have a clear, documented recommendation and approval paper trail before any action is taken.
A financial approval flow is typically in place to prevent fraud, though even that is not foolproof as crooks can be very ingenious and find ways to circumvent the system. The OP has not indicated whether this was a genuine mistake. As Dinesh has pointed out, an investigation should take place before any sanctions.
If this turns out to be a genuine mistake, then you have already subjected this employee to a great deal of distress, apart from his own remorse over the error.
Personal Experience in Purchasing Functions
Disclosure: Nearly all the jobs I have held in the last 40 years involved purchasing functions, including one job involving spending upwards of $65 million a year, so I have a very good knowledge of this, and I have never failed an audit of my work. However, I should point out that in many places, the staff disliked me because I never deviated from the rules and did everything by the book!