You could make it respond to multiple codes, so that you can hand out different codes to different realtors. It logs the codes. The purpose being that if someone hands their code out and an incident occurs, you know which realtor is responsible.
You could also do an SMS link, you call the owner and they'll send a message to unlock the door for you. Of course the owner must be available (cell phones are a good thing). You could set it up so that the owner can send a message that it should unlock for a limited period of time too, basically you set up an appointment to show up there between 2 and 4pm and he sets it so "159" will unlock it during that period.
In fact you don't necessarily need a communication link. It could keep time and all codes are calculated from a seed. So he could type the time they want into his computer and say, "ok, 856 will open it between 2 and 4pm".