My neighbor added 10 truckloads of dirt to their yard and now our backyard floods every time it rains.

Location: Minnesota

For context. The house is rented out and we get along with the tenants. I have never met the owner, nor have I had any dialogue with him. My wife and I have lived here for 5 years. 3 years ago, we added a fence for our dogs, and I attempted to contact the owner via text to show him pictures of our planned fencing materials and ensure him we were marking property lines, etc.
There has always been flooding in our backyard. About a year ago, a small bobcat type of truck and 10 massive truckloads of dirt mixed with stones and sticks was dropped off at our neighbors house. Not only did the dirt encroach into our property, but it also covered the bottom boards of our fence. After this winter, we had a ridiculous amount of flooding from the snow melting.
On our side, we decided to add in a rain garden. We dug 24 inches deep, added sand, pea gravel, dirt, plenty of water absorbing plants and decorative rock. I also changed the direction of the gutters on my garage so it flows away from the rain garden completely. Unfortunately we are still getting, at some points, up to 6-10 inches of standing water in our backyard, which also flows into our garage.
We reached out to the city of Minneapolis, and they came out to look at the property. We found out that indeed, yes, the owner did not follow city rules and regulations, and had been fined. After a month, the owner has still not paid the fine, and the city informed us that it will be placed on the owners property taxes. We took from this that the owner, who owns several units around our neighborhood, couldn’t care less about the fine or our complaints to the city.
Suffice to say, our garage, our rain garden and our backyard have been SAVAGED by standing water. Our sump pump has been running non stop. We finally snapped and reached out for legal help, and have hired a lawyer. We have dozens of pictures, several with a measuring tape in the water, along with clear runoff paths from their yard to ours and the standing water in our garage.
The lawyer wants a retainer of $3000, and we are suing the owner for damages to the garage, which we will need assessed by a civil engineer, and either a permanent fix to their backyard following the city’s rules, or removal of all the dirt.

To be clear, we want a permanent fix, we aren’t trying to screw the guy over. We tried all other avenues, but still find ourselves anxious every time it rains.
Are we doing the right thing? Is the retainer too much, or a good deal? Is hiring a civil engineer not needed? We want to trust the lawyer, but we have no clue what we are doing. Please help, any information would be amazing! I’ll answer any other follow up questions as soon as I can. TIA!!!

TLDR; lawyer wants $3,000 retainer to sue owner of the house (our neighbor) for damages to our property after assessment by a civil engineer. What do we need to know or be prepared for?!