VIC Retaining wall removal

[Retainingwall]

I live in a new estate and moved in8 months ago at which time I was advised I had to put in a retaining wall at some costs. Same builder has now excavated vacant land next door which is comparative to my cut and advised that part of the retaining wall is to be taken down. they advised they would pay a fraction of the cost of which they advised it was the benefit of both parties. What are our rights in this regard as it appears builder has made an error on there part