Is it permissible to waive one's disability pension in exchange for a lump sum from the Social Security Fund, and is this considered usury (riba)?

If the social security salary is a grant from the social fund, and not in exchange for a subscription, then there is no harm in taking the thirty thousand and withdrawing from the fund. This is because a grant is not owned except by taking possession of it, and since you have not yet received the salaries, you do not own them. Consequently, waiving the receipt of these funds is not an exchange of money for money; rather, it is an option offered by the fund between a deferred grant and an immediate grant.