The first one means "Kacho dicided to have Yamada work after worktime" while the other one means "Kacho had Yamada work ~"
You can't judge if Kacho actually did it in the first sentence.
ことにする means like "decide to do it" or "let it be supposed to do it"(I don't know what I mean. haha)
Anyway, 日本語の勉強がんばってね! わからないことがあったら聞いてくださいね!