Neither classic Hippocratic oath nor the commonly used Declaration of Geneva (https://en.wikipedia.org/wiki/Declaration_of_Geneva) would go against a doctor saying "the recommended procedure to cure your ailment costs one million dollars; I'll be treating you if and only if you pay that", assuming that the treatment honestly is the best option health-wise. What the oath could prohibit is the recommendation of a treatment that's objectively worse (i.e. less effective, or more likely to have harmful or fatal complications) but is much cheaper; while a person can choose (or be recommended) bread, wine or car that's worse but cheap, the ethics frown upon recommending worse health outcomes.
In planned events and traditional health care your argument makes sense.
I was specifically thinking about emergency care or other situations where communication isn't always possible or made in a timely manner.
If a someone comes into an ER and turns out needs say a stent put in, when do you get clear agreement? (caveat I am clearly not a medical professional and this might be a terrible example).
My assumption is largely the hospital does not wait to validate someone's credit to execute what would otherwise be something done for normal retail transactions of similar size.
The ability for a doctor (merchant) to refuse service in certain situations just doesn't exist without violating an ethical boundary.