Is it permissible to continue working with someone who purchases hard currency in installments, and to work with him and accept his financial assistance, knowing that this is considered usury?

For currency trading, it is a condition that possession be taken at the contracting session to avoid the prohibited riba an-nasi'ah (usury of delay). If working with a friend leads to directly engaging in these prohibited contracts or assisting in them, then it is not permissible to continue. However, if it is free from that, then it is permissible, though it is preferable to seek work that is free from Sharia violations. Purchasing a home is not considered a necessity that permits committing what is prohibited.