Is it permissible to sell camera equipment to one's brother and his partner, making a profit from them without their knowledge, given that the selling price is lower than the market price, with convenient installments and free maintenance?

If your husband sells these machines to whoever requests them from him, there is no harm in that, even if he does not inform the buyer of his profit, because he bought these machines and has the right to profit from them as he wishes, as long as there is no deception or ambiguity. However, if your brother and his partner appointed your husband to purchase this machine without their knowledge that he is the seller, then your husband is not allowed to take from them anything more than its original purchase price, because an agent is entrusted.