When purchasing goods from 500 yuan, you must obtain a special VAT invoice before you can deduct the input tax according to the VAT indicated on the deduction sheet. In this way, the output tax of goods from 700 yuan minus the input tax of goods from 500 yuan is the VAT you should pay. However, if you get an ordinary invoice when purchasing goods from 500 yuan, or there is no invoice, it cannot be deducted, and you can only pay VAT according to 700 yuan.