Chack the Asset Management Template Knack has, it kinda has the same idea there.
I might help but I’m not sure if what i think is going to do exactly what you need.
What if you created a Yes/No field called “Available” in the keys object, have it default to No, and have a conditional rule that states: when “property” is blank, set available to custom value: Yes. And when property is not blank, set available to custom value: No .
I’m not sure how you want to assign the key to the property, but one way you can do is to create a table view in ur app that’s connected to the keys object and filer in the data source to only show the available keys. Then add an existing page (the form that you use to assign the key to the property) in a column in the same table.
I’m not currently in front of a computer to tell you exactly what you can do, this is just on top of my head.
I hope it helps