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！