A deduction is allowed from earnings payable out of the public revenue for the employee’s fixed sum expenses in respect of the duties to which the earnings relate.

“Fixed sum expenses” means the sum, if any, fixed by the Treasury as in their opinion representing the average annual expenses which employees of the employee’s description are obliged to pay wholly, exclusively and necessarily in the performance of duties to which such earnings relate.

This section needs to be read with section 330(2) (prevention of double deductions).