Is it obligatory to return the money obtained through an undue discount from a private clinic, knowing that the financial situation has improved and the husband has become a teacher?

You must repent from consuming ill-gotten gains. Repentance is achieved by feeling remorse, abandoning the sin, resolving never to return to it, and restoring rights to their owners. You are required to return the discounted amount to the clinic. It is not necessary to inform the clinic owner or the employees; it is sufficient to return the money by any means. If you do not know the exact amount, then exert effort in estimating the number of visits and the discount percentage until you are reasonably certain that you have restored the right.