Neighbor refuses to mitigate dirt that caused fence to fail

[Gretchen P.]

There is a 6' wood privacy fence at one of our properties. The bottom rotted and began to fail due to 8-10" of dirt built up on the other side of the fence. The neighboring property is also a rental. Each condo owner is responsible for their own fencing/yard. Last September we were told by another property owner the fence was failing, and our tenant sent a picture of the fence bowing, but not breached. We contacted the owner's property management company. They sent us a bid that included not only the shared fence but the fences on the sides and front "wing" fences on the neighbor's property. The management company informed us the owner wanted us to split the entire fence bid. We indicated we would be happy to split the cost of the fence contiguous to our property, but that the dirt needed to be mitigated. The back and forth dragged on, later my tenant texted to tell me his beloved dog had escaped through the fence and was killed. Again we contacted the management company who supplied a new bid, which included the fence separated out and a $1300 retaining wall. There was a notation from the contractor next to the portions "split with the neighbor". We immediately replied we would be happy to pay the portions marked for us as long as the owner was building the retaining wall. The management company replied that the owner wanted us to split the cost of the retaining wall. 

The situation now is that his dirt has spilled on to our property and the tenant has a makeshift fence up. The tenant decided not to replace his dog. We can't just put up our own fence as it will eventually fail, but the property owner is standing firm that we should pay for the retaining wall. The fence was there when we purchased the property. 

We are willing to pay for a fence, but not a retaining wall on someone else's property. What can we do?