How can one return donated funds that have been disposed of, in addition to dealing with unsold bonds belonging to a charitable organization that has closed, especially given that many years have passed and the specific amount or the original donor is not remembered?

As for the bonds and coupons, they have no material value, but one must seek forgiveness and repent for their loss, as they were a trust. As for the amount collected in the name of the association and spent without knowing its exact sum, it must be estimated based on the most probable assumption and paid to a similar charity organization with the intention of purification and repentance, not as a charity. The perpetrator must repent, seek forgiveness, and increase good deeds, for "indeed, good deeds do away with evil deeds." Sheikh Ibn Baz advised to disburse unlawful gains in acts of righteousness, along with sincere repentance, regret, abstaining from the unlawful, resolving not to return to it, and increasing good deeds.