Let me break it down: Suppose I put MY dollar in a soda machine... who does the soda belong to?
A deposit for a mortgage with your missus, with her having to pay a bit off by herself is probably a better analogy.
With the twist the missus can sell the house on you if they feel like it and not give you the deposit back.
Or pay off a fair bit off the house if they chose to keep it for a while, then you have to chip in.