The latest person to find themselves in the crosshairs of Donald Trump's twitter act is prominent Iowa evangelical Bob Vander Plaats. The specific accusation that Trump leveled against Vander Plaats is that Vander Plaats asked for $100K to arrange for Trump to come speak at an Iowa event that he, generously, was willing to do. Vander Plaats' response was that it was Trump, in fact, who was paid the massive speaking fee, and that he (Vander Plaats) got nothing.
As you find more often than not when Trump calls someone else a liar, the facts demonstrate that in fact it is Trump who is telling a bald faced lie. The Des Moines Register asked the event hosts to look at any documents they had for the event, and the company complied:
— Jennifer Jacobs (@JenniferJJacobs) January 26, 2016
Peoples Co. paid Trump $100,000 to speak at Land Investment Expo in Iowa in Jan 2015. Was Bob Vander Plaats' idea, but he got no $ in return — Jennifer Jacobs (@JenniferJJacobs) January 26, 2016
This confirms that Vander Plaats was being 100% truthful - that he set up the meeting as a way for Trump to introduce himself to Iowa voters, and got paid absolutely nothing. His thanks for having done this favor (and earned Trump a hefty speaking fee) is for Trump to lie about him on Twitter.
It's bad enough that Trump is a fake conservative and that he constantly says ridiculous things that are damaging the GOP brand forever. It's bad enough that he constantly covers for Hillary every time she has a bad news cycle. He's also a terrible person and a pathological liar.