Is Working in Medical Billing Halal in Islam?

If the reality is exactly as described in the question—namely, that the primary function of the medical billing company is to act as an intermediary between the patient and the insurance company by submitting insurance claims, following them up, and receiving insurance payments to transfer them to the doctor or hospital then, according to the Hanafi school of Islamic jurisprudence, such employment constitutes assistance and facilitation in a prohibited insurance transaction.

Therefore, the salary earned from such employment would not be considered lawful, and the questioner should seek an alternative source of halal income.