Purchase Invoice Vat 7%

Hi Lubos,
Okay. Thailand Tax law requires us to deduct VAT 7% when paying Advance/Deposit for buying goods from supplier. If we take the full VAT calculation which is 68,507.04 THB, then how are we to contra off the VAT we pay earlier (Advance payment)? We need to submit VAT every month and purchase invoice may takes 3 months to fulfill the full payment.