since the buyer has required the seller to provide the hammer heads customized to its needs upon conclusion of the contract and has paid extra frees for this, the seller is defense that it was not able obtain the hammer heads form Company cannot convince the arbitration tribunal to believe that the performance of the seller falls into the exemption as provided under article 79 of the cisg (exemption for imprdiment) because the seller should have the chance to change the contract terms to avoid this upon negotiating the contract.