Is it permissible for me to sell my shares to a person in installments, then sell them to him whenever the buyer wishes, knowing that the shares do not enter his account, but only the money?

If the buyer authorizes you to sell the shares after purchasing them from you, this is permissible. However, it is not permissible for you to buy them for yourself, because agency in selling means selling to another person, not to yourself, and because this gives rise to suspicion. The prohibition is confirmed if you sell them in installments and then buy them back from the first buyer for cash at a lower price, as this constitutes the forbidden 'riba al-'inah (usury of deferred payment and repurchase). The terms of the agency must be adhered to.