What happened when Sammeal sent that guy to make a truce with Rand? If I remember, Rand refused the truce, but Sammeal thought that he agreed. What happened?

I think Sammeal was just boasting to Graendal or maybe another Forsaken that Rand had a truce with him. He wanted to make himself seem powerful or something.

Sammael was doing this thing called "lying".

he told greandel that becuase he wanted to keep her on edge and keep her under his control

It was a double whammy lie as he was trying to convince Graendal that he had the truce and was Nae'blis