let us assume a flow in simple pipe (air). Step 1: I used mass flow rate boundary condition at inlet and outflow boundary condition at outlet. I calculated pressure values at inlet and outlet in post processing step.

In step 2 , I used the above obtained pressure values as my pressure inlet and outlet boundary conditions respectively.

Now question is I am not getting the same mass flow rate at inlet as in step no. 1

Which pressure did you calculate, static or total from case 1?

At inlet I calculated total pressure and outlet I calculated static pressure