Do the neural substrates behind motivation to retain/dispose of property govern whether certain people view their friends and partners as possessions?
I can't speak to your first question, but I think the second question could be profitably reframed.  The orbitofrontal cortex seems to represent incentive/reward value of primary sensory information and is thus linked to the "hot" decision-making system, which manages decisions where there is a gain/loss of reward at stake or risk--this would necessarily cover property and personal relationships, which are both sources of reward.  So it's less that the part of the brain that manages property may sometimes also manage personal relationships, and more that this particular part of the brain is responsible for representing incentive or reward options, which are relevant to different kinds of rewards, including property and people.

Reference (just a good place to start, really):
Rolls ET, Grabenhorst F. The orbitofrontal cortex and beyond: from affect to decision-making. Prog Neurobiol. 2008 Nov;86(3):216-44. http://dx.doi.org/10.1016/j.pneurobio.2008.09.001