In many areas lotlines will prevail, but if you cross it is malicous tresspass. You admit you crossed the property line, maybe the client did not inform you so, but it might have been obviouse.
Could a reasonable person discern that the property line was near? If not, then you may be able to show you did not intend to destroy property.
Did you have any pictures prior to cutting that shows that it was predominantly on clients side? This could reduce the amount they can go for, being that is was a "boundry tree" which are sometimes concidered owned jointly.
Maybe good hig res picture of the hedge wich growth was being influanced by the assumed vollunteer mulberry
You may do yourself and your client a good turn by offering to plant a good species to replace. Maybe a fastigiate amilanchier?
Trash trees have little bearing. For some people green is good. Go for invasive and it may hold some water.
This USDA page sites good authoirity that it is invasive
http://plants.usda.gov/java/profile?symbol=MOAL
This one shows it as being on the noxious invasive list for KY and VA
http://www.invasive.org/eastern/species/6050.html
Here it is on the invasives list for Montgomery, NJ 2003
http://www.montgomery.nj.us/invasiveplants.pdf
http://www.npsnj.org/references/invasive_plant_list.pdf
This is from Appendix to Policy Directive 2004-02
Invasive Nonindigenous Plant Species
October, 2004
The list includes Morus alba
This list is intended to provide guidance for planting, landscaping and restorations on Department lands, and does not constitute an official list of invasive nonindigenous plant species for New Jersey. No legislation currently exists mandating the creation of such an official list. In addition to enabling legislation, the creation of an official list of invasive nonindigenous plant species will require additional research, a thorough review by the scientific community and governmental agencies, and an opportunity for public comment.
You had no right to cross boundries.
The law tries to "make whole" the injured party.
Your best bet is to make the.
You can take these things to show that you had no intent to damamge the nieghbor, along with the offer to replace with a better speiceis of reasonable price.
Taking in the location and the tendancy for these trees to be seeded by bird excrement, one could argue that a smaller tree with better aesthetics is more valuable. My fastigiate recomendation
Here is a nursery in TN with prices
http://www.shadyvalleynursery.com/a.htm
I was going to paste in the table, but it did not move well.
Amelanchier
*canadensis, Shadblow Serviceberry
Noted for its showy apricot-orange to deep rusty red fall color. Early flowering with subsequent edible fruit.
6 ft. multi-stem 49.00