Excess soil and Retaining wall dilemma

Ok now the dilemma about the excess soil and the retaining wall...

Excess soil - One day we came to check on our land and found some excess soil dumped on our land at the far back right corner. From assessing the surroundings, we believe the builder (Metricon) of our neighbour on the right dumped these excess soil on our land when they flatten the corner to do their outdoor concrete slab. We got no physical proof but we have timestamp photos showing there was no excess soil originally, the land was flat, and the next photo shows excess soil around the time our neighbour's outdoor concrete slab being poured. Pretty coincidence wasn't it? We wrote a complaint email directly to Metricon (after details were given by our neighbour) but as expected they denied and not willing to remove these excess soil (was not appreciated with their response at all). We then contacted Department of Environment and Heritage Protection (EHP) and they advised we can make a formal complaint if we wish but because we checked with Plantation Homes if there going to be any additional cost when they do cut and fill with these excess soils they advised as its not enormous amount of soil there won't be any additional cost so we decided to drop this case and not make any formal complaint to Metricon (lucky them).

Retaining wall - Because of our land level higher than our neighbour's on the right a retaining wall is required. Apparently, when our neighbour were building their home they required to install a retaining wall at the far back corner between our land and theirs as that is a weakness point but then somehow their builder, Metricon, had a 'second plan' to build their house and said a retaining wall is not required at the time. Obviously so then the neighbour can wait for us to build and share the retaining wall cost! While the neighbour at the back did not need to wait for us they installed the retaining wall at their own cost.

Well, because our lovely neighbour on the right want to avoid paying the full cost of the retaining wall and did not do the 'proper way' issues with that corner arises - each time it rains our land soil fell over to their side and washed away, slowly creates a massive hole underneath their outdoor concrete slab (oh boy) as you can see in the below photo. Also because at the time it rains so much that their concrete slab corner started to crack (due to hollow beneath). Of course our neighbour panick and called us for a solution. Unfortunately nothing we could do as the issues are on their land and we said to them 'You should've done the right thing at the beginning - install a retaining wall!'. Jokes. Instead of saying that we said 'We suggest you to contact your builder and ask them why they said to you it is OK not to install the retaining wall when building'. They end up getting no solution from their own builder but to get a shovel and shove our 'excess soil' underneath their slab to cover up the hole and hope its not going to rain again. 

Is this called karma?

Cracked line on neighbour's concrete slab