What is a Phase Delay filter?
The thing you're looking for is known as an "all-pass filter", which has a flat amplitude response, but modifies the phase relationships within a signal. This would be a separate stage from the one that limits the bandwidth.

Note that the 50-500Hz bandpass filter is going to modify phase relationships, too, so the all-pass filter will need to be designed so that the overall system response is what the client is looking for.

However, it's possible that the client just wants a time delay, but expressed it in terms of the phase angle of some fundamental frequency. This will need to be clarified.