A VAT calculation with stated eligibility

A classroom exercise supplies a ten-percent VAT rate and tax-exclusive taxable sales of one hundred million won. It gives input VAT of four million won that is fully eligible for deduction and a separate six-hundred-thousand-won input amount that is expressly nondeductible under the exercise assumptions. No zero-rated sales, exempt-activity allocation, prior balances or additional adjustments are included. A trainee deducts both input amounts because they appear on invoices, while another calculates output tax on the sales plus tax rather than the stated tax-exclusive sales value. The task is to calculate the net amount under the supplied assumptions and explain why the invoice alone does not establish input eligibility.