Must the wife return the 500 dinars with a 5% annual interest so that her previous four years of service are counted towards the social security years, or should she return the amount without interest and those years not be counted?

Social security is permissible under three conditions: the owner's consent, that the security be cooperative and not commercial, and that its funds not be invested in unlawful (haram (definition)) activities. If these conditions are met, participation is permissible; otherwise, it is not. Security schemes whose owners seek to accrue larger sums upon retirement are commercial and participation in them is not permissible voluntarily.