Appointing an "acting-landlord"...tenants refuse to acknowledge

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I'm a landlord of a multi-unit property and all of my leases are between myself and the tenants. However, im moving out of state soon and have a friend willing to take care of rent collection, evictions, and everything else. I'd like to officially appoint him as an agent on my behalf. The problem im having is a few of my tenants are opposed to dealing w anyone except me. their argument is since they signed the lease with me, i'm bound to deal directly with them. Seems like a plausible argument....Looking for advice on this one.

Thanks ahead of time