Make overage provision on selling land illegal

An overage provision is put on land by the vendor, so when new buyer either gets development planning or increases the value of the land the new owner has to give the old owner up to 50% of the value gained on top of what they paid. This is unfair to people purchasing land and wanting to add value to it, if the old vendor wanted to gain from developing the land then they should have done it when they owned it, why should they gain from someone else’s hard work.