884. For the purposes of sections 885 and 886, where an employee or other beneficiary receives, in a taxation year, an amount from a trustee under a deferred profit sharing plan and the employee has made a payment in the year or a previous taxation year to a trustee under the plan while it was a deferred profit sharing plan, the amount determined for the year, under this section, in relation to the plan and in respect of the beneficiary, is such portion of the aggregate of the amounts so received in the year, minus any amount determined for the year under section 883 in relation to the plan and in respect of the beneficiary, as does not exceed the amount by which(a) the aggregate of each amount so paid by the employee in the year or a previous year, to the extent that such amount was not deductible by the employee in computing his income, exceeds
(b) the aggregate of each amount received by the employee or other beneficiary from a trustee under the plan while it was a deferred profit sharing plan, to the extent that such amount was included in computing the amount determined under this section for a previous year in relation to the plan and in respect of the employee or other beneficiary.