Is it Islamically permissible for a paint factory owner to contract with a merchant for the latter to purchase paint at a specific price and within a defined period, with the factory owner giving the merchant a sum of money or its equivalent in goods in addition to the usual discount?

The transaction mentioned is either a salam sale or an ordinary sale. If it is a salam sale, meaning the merchant requests paint with specific specifications to be delivered at a future date, and the capital is paid in advance, then it is not permissible for the factory owner to give the merchant anything as a gift if it is stipulated, because the factory owner is indebted to the merchant. However, if the transaction is a sale with no salam or debt involved, then there is no objection to giving a gift, whether stipulated or not.