Enforcement Rules of Stamp Tax Act Article 9
If the actual price of the contracting agreement depicted in Item 3, Article 7 of the Act cannot be determined until the completion of work, the taxpayer shall affix the tax stamps for estimated tax due at the time the contract is entered and then make up the shortage or apply for a refund of excess tax paid when the work is completed.
中文原文 印花稅法施行細則第9條
本法第七條第三款之承攬契據，如必須俟工作完成後始能計算出確實金額者，應在書立後交付或使用時，先預計其金額貼用印花稅票，俟該項工作完成時，再按確實金額補足其應貼印花稅票或退還其溢貼印花稅票之金額。