Is a savings certificate with a fixed 15% monthly profit, which cannot be redeemed before a year except with a deduction from its value, Islamically permissible?

Investment certificates with a fixed annual profit (15% of the certificate's value) are not permissible for investment because they constitute a loan with usurious interest, as their profit is an increment on the principal. This certificate is not considered a legitimate Mudarabah (profit-sharing partnership) because the profit in it is predetermined as a percentage of the capital, whereas a sound Mudarabah requires the profit to be shared between the two parties as a percentage of the profit, not of the capital. It is necessary to ascertain the type of certificate and whether the transaction is with an Islamic bank that adheres to Sharia controls. As for investing with conventional, interest-based banks, it is forbidden, and any increment on the principal is usury that must be disposed of by giving it as charity to the poor.