The doctor is also in a business to earn money. The doctor has to pay all kinds of bills and incur expenses. If he doesn’t charge a reasonable fee, soon he will be out of business too. If he starts providing free services, soon more and more people will come to him in dire situation, eventually the doctor will have to reject services to some. So what is your suggestion the doctor should do.
If we look from business perspective, it is absolutely Halal that a doctor reject services unless his fees are paid. If he provides free services to some, it is just a charity work.