Is it permissible to deal with a Christian female merchant when a Muslim female merchant is available? Is trade recommended? Is it permissible to deal in installments? Is there a specific profit margin in trade?

The basic principle regarding transactions of buying and selling with non-Muslims is permissibility. Indeed, the Prophet, peace and blessings be upon him, interacted with Jews and bought food from a Jew. The ruling on the permissibility of trade varies depending on individuals and circumstances: earning is obligatory for the needy, sunnah for those who have sufficiency, permissible for increasing wealth and expansion, and forbidden if it leads to harming one's religion or honor. Selling by installments is permissible, and there is no objection to increasing the price of the commodity in such a transaction as long as the price is determined and known at the time of contracting. There is no specific profit margin mandated in Islamic law; the seller may determine the profit margin they desire as long as there is no cheating or deception of the buyer.