Is taking $700,000 in exchange for cashing a $1,000,000 banknote considered usury (riba) or a commission, and is it permissible to mediate in such a transaction while doubting the source of the banknote?

If a financial paper (check or promissory note) is sold for less than its value, it is Islamically forbidden (haram) and it is not permissible to mediate in such a transaction. As for paying a sum for mediation and using one's influence and connections to obtain a right, there is no harm in it, though there is a difference of opinion among scholars. Regarding assisting the owner of the paper when there is doubt about its source, it is judged by its apparent ownership, but if strong evidence indicates otherwise, then one refrains from assisting him.