Is it permissible to exchange a portion of land that its owner designated as a waqf (endowment) for two other doors in commercial shops that he owns, after selling the original land and informing the buyer of the intention of waqf, and after the buyer built upon the land such that it is no longer possible to register the waqf in the seller's name?

A mere intention, not accompanied by words or actions, does not constitute an endowment (waqf). Rather, an endowment is constituted by explicit statements indicating it, such as "habs" (detainment), "sadaqah (definition)" (charity), "waqf" (endowment), and any other terms that imply it.

Therefore, the endowment was not constituted by mere intention. It is recommended for the man to implement the good he intended, because an endowment is an ongoing charity (sadaqah jariyah). Since he sold his land, he is not legally obligated to compensate for what he intended as an endowment, but it is recommended for him to do so if he is able. If he endows from his commercial shops, that would be a new endowment, not a compensation for the first one.