按照合同，偷工減料不是"default"

There is usually a default clause in the contract. You can withhold fund only when the default clause is triggered. 

Whatever is "default", I don't think 偷工減料 is one of the conditions.

The legal standard is "substantial performance".  Whether 偷工減料 is below the standard of "substantial perfomance", is a matter of facts.

So where is your evidence? 

這個案件法學生都讀過：https://en.wikipedia.org/wiki/Jacob_%26_Youngs,_Inc._v._Kent