Are the funds that I obtained from the Engineers' Syndicate, which were much greater than what I paid for my subscription to the healthcare project, permissible or impermissible? And if they are impermissible, who is the rightful owner to whom they should be returned?

The gist of the answer:

It is permissible to subscribe to the healthcare project affiliated with the syndicate if it is cooperative insurance not intended for profit, as it is considered a contract of donation and cooperation in bearing losses. It is permissible for the subscriber to receive more than what he paid, as ruled by Sheikh Ibn Uthaymeen and the decision of the Council of Senior Scholars.

If the insurance is commercial, it is permissible out of necessity.

It is forbidden to subscribe if you know that the syndicate invests the subscribers' funds in usurious investment accounts.