Is it permissible to sell goods on credit without knowing their exact cost, or to sell them before pricing them and preparing their invoices?

For a sale to be valid, the price and the item sold must be known with a degree of knowledge that prevents egregious ignorance leading to dispute. If the ignorance is minor and does not lead to dispute, then the sale is valid. If the price is semi-known to the contracting parties, and the possibility of increase or decrease does not usually lead to dispute, then there is no harm in not specifying it at the time of the contract, especially when there is a need for it. However, it is preferable to specify it to avoid dispute.