When TOIL* is requested, the amount specified is added to the user's allowance. TOIL does not deduct from the user allowance, it reimburses the allowance instead.
Once TOIL has been approved, the user can then request leave (as normal) and the amount will be deducted from their allowance.
For more information on what is TOIL and how to set it up, please see here.
* This may have another name within your organization.