Is the remaining amount for the cost of the X-ray considered a debt owed by the inquirer, given that he paid for the examination and medications on the same day?

For the validity of a rental contract (ijarah), mutual consent (taradi) and knowledge of the benefit (manfa'ah) and the wage/rent (ujrah) are required. If the ijara was agreed upon but the wage/rent was unknown, then the ijara is void (fasidah), and the wage of the like (ujrat al-mithl) becomes due.

So, if you consented to undergoing the X-ray and knew that the doctor performed it without the price being known, then the wage of the like becomes due upon you, which is what experts estimate (the customary price). However, if you did not consent to undergoing the X-ray, then nothing is due upon you, because whoever works for another without their permission is not entitled to a wage, unless there are exceptional circumstances, and your case is not among them.