Is it permissible to complete a contract for the sale of land that was purchased from a seller who has not yet finished paying its price, what is the ruling on the profits resulting therefrom, and does the absence of a specified date in the contract invalidate it?

It is permissible for someone who has bought land but has not paid its full price or a portion of it, to sell it to another, unless the contract stipulates a prohibition on selling before the price is paid. The fact that the land is not registered in the buyer's name does not harm the transaction, as registration is solely for documentation.

If the contract stipulates a prohibition on selling, it is not permissible for the buyer to sell the land except with the seller's permission. If the buyer sells the land while prohibited from doing so, the sale is invalid and must be annulled, unless the original seller grants permission.

If your purchase from the merchant was valid, you may sell the land unless you are prohibited from selling it, even if you have not paid its full price.

As for deferred sales, the majority of jurists require the term of deferment to be specified. If the contract is concluded with an unspecified term, jurists have differed regarding its ruling, oscillating between invalidity, nullification, and validity. Therefore, if you can rectify the contract by mentioning the time of transfer or payment of the remaining amount, do so, to avoid scholarly disagreement, or promptly pay the remaining sum.