I work with a lot of investors that won't necessarily list the house for lease with me, but offer to pay me a leasing fee any time I bring them a tenant that leases. This type of arrangement is generally with an investor that manages their own rental units.
Any time that I have someone looking for something that I don't currently have available (either listed, personally owned, or in MLS), I go to the phones and start calling investors that may have a vacancy. It gets me a leasing comm, it keeps me in touch with my investors, and it keeps their investment properties full....Win/win deal for everyone.
Fees are always negotiable. You'll find that in some areas it is customary to base the fee on that total rent, other areas it may be based on 1 month's rent, or it could just be a flat fee. You'll want to talk with your broker about that.
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No legal advice given......And my opinion is worth exactly what you paid for it.