Is it permissible to place dollars in a fixed-yield bank certificate to offset the difference in the dollar's exchange rate, then stop it after recovering the true value before its price decreases?

Regular investment certificates with a fixed, guaranteed return are usurious loans forbidden [in Islam]. It is not permissible to invest money in them. Whoever has done so, it is obligatory for him to stop immediately and seek a permissible investment. And whoever gives up something for the sake of Allah, Allah will compensate him with something better.