بسم الله الرحمن الرحيم
(Fatwa: 904/904/M=09/1436)
If it is established that the said person had endowed his land for the mosque then it became waqf for mosque. No matter if the land was not taken into possession, it is enough to make verbal waqf. It is not lawful to sell the endowment after it is finalized. In the above mentioned case it is not lawful to sell it. And in case the condition of exchange is not met then it is also not lawful to exchange it from another land. The ruling is mentioned in details in Shami etc.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband