Is it permissible to reclaim a waqf (endowment) land after the financial circumstances of the donor have deteriorated, especially since the the waqf was neither registered nor acted upon thereafter?

It is not permissible to revoke an endowment (waqf) if you have explicitly or implicitly declared it, or performed an action indicating it with the intention, because it leaves the ownership of theenderower. Your poverty is a tribulation for which you will be rewarded with patience, and you have the right to claim maintenance or take from the zakat.