Is the Social Solidarity Fund, to which teachers contribute a monthly amount and receive multiple times that amount at the end of their service, considered a permissible reward, or does it involve an element of usury?

If the amount paid at the end of service is the sum of what was paid to the employee and what the employer added as a grant, or if it resulted from investment in permissible projects, then it is permissible, and it is permissible to continue in the fund. However, if the amount includes usurious interest, or if it was invested in forbidden matters (such as alcohol and cinema), then the portion resulting from the forbidden activity must be disposed of by spending it on the welfare of Muslims, and it is not permissible to continue in the fund.