why did Farfrae and Henchard decided to dispense eachother?

Competition, they both loved the same woman.

Because when Henchard hired Farfrae as his business manager, he did way better than Henchard and everyone started praising him. No one would value Henchard, even his daughter Elizabeth Jane started to court him secretly, so Henchard became jealous of Farfrae's success and fame and decided to dispense with him.