You can go ahead. I need to stop by at the general affairs department.
You can go ahead.
先に行っててください。
I need to stop by at the general affairs department.
総務課に寄っていく必要があるから。
もちろんシンプルに"I'll go stop by at the general affairs department."としても良いですね!
stop by:立ち寄る
general affairs department/section:総務課