Your question implies a lack of understanding of how negative feedback works with a reference to the desired common mode midscale when the input differential is null. The current source is controlled to regulate the common mode gain using the output.
An Op Amp uses the output to null the inverting input for negative feedback to track the non-inverting input if using a scale factor (Rf/Rin) to null the error so that the non-inverting input is equal to the output with a gain of 1+ Rf/Rin.