Amount of the withholding.

Regulations:article 31.2 Non-Resident Income Tax Law

The withholding must be an equivalent amount to the tax owed deriving from the provisions of the tax itself or as established in an agreement.

However, the withholder should not take into account the following expenses or deductions (which are applicable on calculating the tax):Deductible expenses, the liability of the special tax on fixed assets belonging to non-resident organisations and the deduction for donations.