Thank you for providing your state name, JohnLittle.
Slander, by the way, is oral or spoken defamation. Libel is its written equivalent.
Anything you say about your neighbor to your neighbor
alone, when no one can hear what you say, is not slander. For a statement to be defamatory, it must be communicated to a third person (at least one other person). So you can call your neighbor a cheat or a crook or a slumlord to his face without worry of a defamation suit. In that situation, the worry comes from the risk your neighbor's fist will wind up in your face.
What you WRITE online, however, carries more legal risk. Libel is, for one thing, more permanent than oral comments. Libelous statements stick around while slanderous words are fleeting. So you must be careful not to write false comments that harm the character and reputation of your neighbor.
Words like cheat and crook and slumlord are considered "red flag" words that should be avoided. Words that can cause harm to a person in his trade, occupation or profession are especially dangerous to use. They can support, and have supported, costly defamation lawsuits.
If you feel the need to write about your neighbor, stick instead to provable facts (his tree fell on my car, he refuses to pay a fair price) and PURE opinion (I don't like him). You should not state or imply anything false.
It can be important to remember that "truth" is only a defense to a defamation claim. It does not prevent a suit.
If you want to ignore your neighbor's offer to pay you $400 in compensation for damage to your car in exchange for your silence, you are free to ignore it. Trying to buy silence never works very well anyway.
I suggest that, instead of a battle of words (online or off), and if an amicable agreement cannot be reached, that you get the insurance companies involved or you take your neighbor to court and sue him for damages.
Good luck.