Is purchasing government investment certificates (Shahama) in Sudan considered usury (riba)?

Islamic banks and companies have replaced investment certificates with Muqaradah bonds (Sukuk (definition) al-Muqaradah), which are an investment tool based on dividing the Qirad (Mudarabah (definition)) capital into equal-value ownership certificates representing common shares in the Mudarabah capital. The Shariah legitimacy of these Sukuk requires that the certificate represents common ownership in the project, that the prospectus specifies the contract terms, that they be tradable after subscription while observing the rules of currency exchange, debt trading, and asset disposition, that the subscriber be the Mudarib, and that their trading in financial markets is permissible. The Sukuk may not guarantee capital or a fixed profit, nor compel a future sale, nor stipulate a specific share of profit for the company or a determined amount for the Sukuk holders. Profit becomes due upon its appearance and is owned upon liquidation or valuation. It is permissible to distribute proceeds before liquidation as interim payments, and it is permissible to deduct a percentage as a reserve to cover losses. Furthermore, it is permissible for a third party to volunteer to compensate for losses in the project.