What is the best way to return stolen money from the state to its rightful owners, with the possibility of investing and developing it to return the full amount, and are the profits generated from this investment lawful (halal) after the full amount has been repaid, and is it true that earning from trade with unlawful (haram) money is 50% lawful?

It is impermissible to transgress against public funds. Whoever does so must repent and return the money from where it was taken. If it is impossible for him to return the money in the same manner, he should place it in the public expenditures where the state spends. As for the profits generated from investing the stolen money, there is a difference of opinion regarding them. The most just opinion is to divide the profits between the thief and the state, similar to a mudarabah profit, such that the thief is given what a mudarib would receive if the contract had been a mudarabah agreement, and he is obligated to return the remainder along with the principal. Repentance is immediately obligatory, and it is not permissible to delay donating the money for the purpose of investing it unless he would be harmed by doing so. He must donate what he can, and the remainder becomes a debt upon him.