This section applies where—
by virtue of an agreement between a worker in retail employment and his employer, the amount of the worker's wages or any part of them is or may be determined by reference to the incidence of cash shortages or stock deficiencies, and
the gross amount of the wages payable to the worker on any pay day is, on account of any such shortages or deficiencies, less than the gross amount of the wages that would have been payable to him on that day if there had been no such shortages or deficiencies.
The amount representing the difference between the two amounts referred to in subsection (1)(b) shall be treated for the purposes of this Part as a deduction from the wages payable to the worker on that day made by the employer on account of the cash shortages or stock deficiencies in question.
The second of the amounts referred to in subsection (1)(b) shall be treated for the purposes of this Part (except subsection (1)) as the gross amount of the wages payable to him on that day.
Accordingly—
section 13, and
if the requirements of section 13 and subsection (2) of section 18 are satisfied, subsection (1) of section 18,
have effect in relation to the amount referred to in subsection (2) of this section.