He sold land to someone for a deferred payment, and that person sold it for a higher price; can the transaction be cancelled because of the loss?

If the sale has been completed and the purchaser has taken
possession of the land, it is permissible for him to sell it for the same
price or for more or less, because it has now become his property that he
may dispose of however he wishes, by giving it away, selling it, renting it
out or otherwise. The fact that it has not been registered in his name does
not matter, because registration is only for the purpose of documentation.
It also does not matter that he had not paid off the balance, because by
means of the transaction it became his lawful property and he is to be asked
to pay off what he owes, unless you had stipulated that he should not sell
it until he had paid off the outstanding balance, or if the reason for not
registering it was so that he would not be able to sell it until he had paid
it off. This is a permissible condition and means that the land is being
held in pledge until the price is paid in full, and he has to adhere to the
condition and it is not valid for him to sell it to a third party before
that. 

But if you did not stipulate that he should not sell it,
there are no grounds for not allowing him to do so, because he may dispose
of his property. 

You do not have the right to try to cancel the transaction
out of jealousy of the profit that he made by selling it, unless you were
deceived concerning the price for which you sold it, in which case you have
the right to cancel the transaction or go ahead with it. 

According to some fuqaha’, this is called khiyaar al-ghabn
(option of cancelling the deal in the event of gross and unfair
underselling)  

There are several forms of khiyaar al-ghabn, such as: bay‘
al-mustarsil, which refers to one who does not know the value of things
or is not good at bargaining, so he may buy or sell something for far more
or less than it is worth according to people of experience. In that case he
has the choice. But if he knew the value of things and he is good at
bargaining, then this is not applicable. 

If the option of khiyaar al-ghabn is not applicable in your
case, you cannot cancel the transaction, because the sale is a binding
contract unless he lets you off and agrees to cancel it. 

If you stipulated that he should not sell it until he had
paid off the price, then he is obliged to adhere to that and is not allowed
to sell it until he has paid it off. But you do not have the right to cancel
the transaction with him except with his consent, because of what is stated
above about the transaction being a binding contract. 

And Allah knows best.