Realtor Fee - 1 Month's Rent?

[Joel Heck]

Hello,

I'm looking to purchase a duplex in Michigan I found that is currently rented. Looking over my realtor's broker agreement, I noticed that in addition to a typical 3.5% commission fee, there's a "rental fee" for which I would have to pay him one month's rent since it's an investment property. I haven't seen a fee like this anywhere in the forums or blogs. Is this typical? 

Thanks