Is it necessary to ask permission before including someone in the acknowledgements of a economic research paper?
In short, no it is not necessary. I have never asked anyone for permission and I have never been asked. The people I thank usually have not read my paper (rarely anyone does, to be honest), but we have had chats on visits or after seminars or conferences. If you thank someone make sure they are familiar with the paper. Certainly include someone who presented a formal discussion at a conference.
By all means, never include someone in your thank-you note who you have not talked to. People will see this as a strategic way to influence referee selection. However, if someone made great comments that greatly influenced the direction of the paper, name them. It's free and makes people happy.