All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Since these people gave you permission to bury your father in their land, they should not go back on their word as this part of the land had been allocated to the dead. However, if the dead is decomposed, they can in this case benefit from the land. Imaam Al-Shaafi'ee said: 'If a person gives permission to people to bury in his land, then wants to go back on his word, he can do so, as long as they did not bury in it, but he cannot take back the part of the land in which they had already buried a dead person.'
Ibn Qudaamah said: 'If the dead is decayed, then the owner of the land can take his land back.'
Therefore, the grave of your father does not eternally belong to him. And the owners of the land have no right to take it back before the corpse is decayed. Nevertheless, if the landowners want to benefit from the land before the corpse is decayed, and you are not able to prevent them, then you can move the remains of the dead. Whether the dead is decayed or not is a matter that can be determined by the people who are specialized in the field in your country, as this differs from one country to another.
Finally, we remind you that it is disliked to write on one's grave. Al-Bukhari and An-Nasaa'ee reported that Jaabir said: "The Prophet forbade building over the grave, plastering it, extending it, or writing on it."
An-Nawawi said: (Al-Shaafi'ee and the scholars of his school) said: 'It is disliked to plaster the grave, or to write on it the name of the dead, or the like, or to build on it', and there is no difference of opinion about this in our view. This is also the view of Imaam Maalik, Ahmad, and the majority of scholars. However, Abu Haneefah said: 'Our view is that it is not disliked.' We do not agree with this statement and our evidence is the previous Hadeeth.
But it is allowed according to the majority of the scholars to put a sign to mark the grave by using a stone or (a stick of) wood or the like, and it is desirable according to the Shaafi'ee school.
Allaah knows best.