Hiring a lawyer does not mean you own the lawyer. $500 probably does not buy much of that lawyer's time. The lawyer can quit the case, if the lawyer provided less than $500 worth of time, then the lawyer might owe you a small refund. If the lawyer provided more, you may owe the lawyer more money. It does sound like you need a lawyer.
This response does not create an attorney client relationship and is offered for informational purposes only. Only a lawyer fully versed on the facts and circumstances of your case can properly advise you on the case. I am licensed to practice in Minnesota, not every state. You should always consult with an attorney licensed in your area on how best to proceed.
I just answered a question of yours on this topic with virtually no factual information. I now see another question with more facts. I suspect the lawyer said "I quit" if you take the gun because even though you say it was not used in the crime he has an ethical obligation to preserve evidence, even if given to him in confidence. If you wanted to take the gun back, the lawyer may have been concerned about either spoliation of evidence or being part of a conspiracy after the fact. I would have done the same thing. As for the $500, I suspect he spent at least that amount in the work and time he put in for you. You can always report him but I doubt it will go anywhere.
It sounds like the lawyer has contacted the Detective several times. Given he has spent time on that, the time he has spent consulting you, and any other investigation into the matter, it sounds like he has done enough work to justify his fee. It is also my experience that police detectives are busy, and picking up the gun might not be a priority if he is working another open case.