What if we found shear slump instead of true one? Should we reject the materials or we have any remedies?

SHEAR Slump values should never ever be taken for any purposes. A fresh sample has to be tested.

If it is possible to get true one in another location, then it is advisable to reject the materials, but if not it's better to go for remedy by thorough washing which requires more time as well as adding more additives to increase cohesion

we will re test for slum if again same then reject

Slump can affect the workability, strength and durability of concrete. Some remedies like adding water can be done as long as it is within the specifications.

http://www.qemsolutions.com/news/a-simple-guide-to-concrete-cube-testing

Shear slump indicates that the concrete lacks cohesion. It may undergo segregation and bleeding and thus is undesirable for the durability of concrete. Test materials and use of possible additives.