Can a former business partner change a verbal property agreement after the fact?

Based on your narrative, if he signed the titles to the trailers over to you, and you titled them in you name, there is not much he can do, unless he obtains a court order.  It is unlikely he would be able to convince a judge that you breached your agreement.  If you purchased his share of the semi-trailer, keep a copy of the checks/receipt in a safe place, in case you are challenged.  It does not sound as if your need an attorney at this time. This sounds as if the seller now has second thoughts about selling the semi-trailers.  He is just trying to extract more money from you, and based on what your wrote, he has no basis to do so.