,

This is a strange phrase that did not score any Google hits, so your Real Estate Co. could just be the only one on earth offering this service!
Everything tells me that procedencia de los inmuebles is about checking legal title to property, usually done by a lawyer, but nothing stopping an Estate Agent doing it, as many are now teamed up with lawyers.
Adding "clients" to the term, I must admit, has got me stumped, but it could mean simply that it is verification of vendors' (who are clients of the Agent) title deeds.

another option!