Like I said, your implementation would depend on context, but to adapt my example, I'd probably put a function on the Property class to determine which Contract class to use - something like:
However, I think from what you're saying that your sell_property() function would need awareness of all property types, so adding a property type not only requires changes to the part of the code which contains the property data structure, but also to the sell_property() function - and any other function throughout your code base which uses properties. That is of course possible, but OO does make it easier to find where to implement logic specific to different property types.
As I said, I'm assuming that the operation depends on the types of _all_ objects involved. So what you actually want is multimethods.
In an OO system you have to simulate multimethods by chaining the calls through all objects, but that obscures the actual functionality of the operation. OO works well if method resolution depends on exactly one type. That's why I chose an example where it's not clear that one type dominates.
In the absence of multimethods I find that 95% of the time a simple if else construct does the trick just fine. But I do value the OO style single type dispatch where it really fits. It is just overused.
