Hi
Once the Donor has gifted the land, it is true that the WAQF board cannot sell the land allocated for building a mosque generally.
However if there is already a mosque(fully functional mosque with prayers being held all through the year) and you are only acquiring the nearby unused land for a specified purpose (without any detriment to the devotees or the functioning of mosque) , then the WAQF board can sell the property to you at market rates subject to 2/3rd of its board members approving the sale of the property.
In your case, the WAQF should also display the notice of sale in the notice board of the Mosque for a period of 1 month and invite objections from public and should public have any grievance, resolve to address the grievance.
So legally and technically WAQF board can clear the sale of land to you subject to
a) 2/3rd of board members expressing their consent in writing for sale of the property and
b) The sale should be at Market rates as defined by the Government (Sub Registrar office) and
c) Display of intention of sale being made at Mosque and grievances if any being addressed.
Hope this information is useful.