My 3 year old Malamute killed my neighbors chihuaha what should I do? ?

So on sunday, my husband and I took the kids and our two female dogs ( 3yr old malamute and 5 yr old american eskimo) for a nice walk. When we came home my husband took our dogs leashes off to get them in the back yard, when all the sudden my neighbor's 2 chihuahua mix came running to attack my smaller american eskimo, and then my malamute took charge and went to defend her,( this was all very scary and surprising to me because my malamute is the swetest dog, she loves people and was never aggresive to other dogs, although she has killed rats and opossums, but never showed any signs of aggresion to other dogs). Anyway the fight like you may have expect didn't end up good, the lil chuhuaha that my malamute fought got what we saw at the time a punture wound that our neighbor said it could be bandaged, both me and my husband apologized to him, and he also said it was his fault for not having his dogs on a leash. I called him on monday morning to ask how his dog was doing and he said that she was in pain but seemed ok, then I get a call today from him saying that he ended up having to put her down, because apperantly the lining for her intestines had ruptured along with some other internal injuries that would had cost him over 2500 to try to save her life which the vet said that may have not even save her. I feel soo incredibly bad about this, I cannot believe my dog did this, I offered to help as much as I could to help him pay the vet bill that was $250 but other than that I don't know what else to do.... Should I have some kind of recipt for the money I give hin, just in case he tries to sue? My husband says that even if he did he wouldn't win because it was his dogs that attacked our dogs.. But I'd rather be safe than sorry... Ugh does anyone have any advise? And what should I do about my dog