Is it permissible to pay money to the Social Security Fund to receive a retirement pension and guarantee medical treatment, given that I reside in Tunisia and do not know how the Fund invests these monies, and that most banks in my country are usurious?

There is no objection to subscribing to social security funds if they adhere to Sharia (definition) guidelines. It is necessary to inquire about the fund's system, the nature of its operations, and how it invests money. If the aforementioned conditions are not met, then subscription is not permissible, and similarly, if one does not know the nature of its operations.