Is paying a portion of the customs reduction commission to government employees considered a bribe, and is money earned thereby unlawful? Is taking a full financial bonus through a fictitious invoice to avoid taxes unlawful? And how can money earned thereby be purified?

It is not permissible to take a commission for reducing customs duties. If customs duties are levied rightfully in return for services, then it is not permissible to evade them, and what is paid to evade them is a forbidden bribe. If customs duties are taken wrongfully and unjustly, then it is not permissible to take a commission for repelling injustice. However, it is permissible for the wronged person to pay money to reduce them or evade them. In this case, what is paid is a bribe on the part of the recipient, not the giver. As for providing an invoice to evade taxes, its ruling depends on the ruling of levying taxes themselves. If they are taken rightfully, then it is not permissible to evade them. If they are taken wrongfully, then there is no harm in evading them. To purify wealth from unlawful earnings, one must estimate the amount of unlawful money that has entered it, and then remove the amount that is most likely unlawful and return it to its rightful owners if they are known; otherwise, it should be given as charity on their behalf. As for what is due to the state, it should be spent on the welfare of Muslims.