Is the amount offered in exchange for signing and approving the sales contract rightfully mine, and how much of it is permissible for me to take?

If you are granted usufruct rights over land without a lease contract, you are not permitted to sell it or rent it to another party except with the permission of the granting authority, because you only own the right of usufruct, not the usufruct itself (the benefit). Al-Qarafi said: "Granting usufruct is for personal direct use, whereas granting the usufruct [itself] includes direct use, renting, and lending." Al-Adawi and Al-Suyuti stated similar views.

Secondly, if the granting authority has specified the usufruct for cultivation, it is not permissible to use the land for construction, as this constitutes a breach of the condition and an encroachment on the property of another. Allah (SWT) says: "O you who have believed, do not consume one another's property unjustly but only [in lawful] transaction by mutual consent." [An-Nisa: 29] to His saying: "And whoever does that in aggression and injustice – then We will drive him into a Fire. And that, for Allah, is easy." [An-Nisa: 30]. The Prophet (peace be upon him) said: "Indeed, there are men who delve into the wealth of Allah without right, and for them is the Fire on the Day of Resurrection." And, "Muslims are bound by their conditions in what is lawful." Based on this, you do not have the right to the amount offered for selling the land for construction, and if you rented it out without permission, you must reclaim the land and return what the tenant paid.