All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Among the conditions for the validity of the sale, is that the goods to be sold should be in the possession of the seller –or that he is given permission to sell it –so, if the sale is conducted between you and the owner of the land with Ijaab and Qabool (i.e. the positive proposal and the acceptance) and the option to cancel the transaction between you and him is no longer existent, then the land is transferred to your ownership and it became your property whether or not you registered the land on your name. Therefore, it is permissible for you to sell it even if you have still to pay some installments from its price to the first seller as long as the sale is concluded between both of you. The fact that you are still obliged to pay part of its price does not mean that the deal is not concluded or that the deal is not valid.
However, we draw you attention to the fact that if the land is not registered on your name, and you are not able to hand it over to the person to whom you want to sell it, then it is not permissible for you to sell it because among the conditions of sale is that the sold goods is possible to be handed over. That is because the goods that one is not able to hand over is like that which is non-existent, so its sale is not valid, and the Prophet forbade the sale in which there is deceit in order to protect the money from being lost and in order to prevent disputes between people.
It is for this reason that the jurists forbade the sale of a stray camel or horse, or birds in the sky, or fish in the sea because it is not possible to hand them over. Therefore, if you are not able to hand over the land which you mentioned in the question to the person who wants to buy it from you, then it is not permissible for you to sell it.
Allaah Knows best.