I can almost guarantee to you that if you just clean the carpets and nothing else gets cleaned in this property, the smell will remain. It just gets into the whole fabric of the building.
Tell the agent the only surefire way to deal with this is to take down and clean (or throw away) any curtains/drapes etc or anything made of fabric, including divan beds. Then the whole place needs a couple of coats of paint before you go in and clean the carpets and upholstery if there is any. Fogging might be an option before decorating, as a bit of a belt and braces approach, but depends on cost.
Clean with a colloid (m-power/nemesis) mixed with deodoriser, put lots on and give it a very long dwell before extracting. Then afterwards re-spray it with the same solution, brush in and leave. If that doesn't fix it, new carpets!