T'Pring becomes Spock's property because she challenged Spock and lost and is now obligated to give him absolute obedience. And then in a twist, Spock gives Stonn back to Vulcan ("it is possible but has never been done before" says T'Pau) and then "gifts" T'Pring to Stonn as his property with the requirement of absolute obedience. "See that you exact what is due to you - as her absolute owner."
Much of this was removed from the aired version. Vulcan is a very harsh patriarchal world. And Vulcans know how to exact their due when their beloved captain is murdered.
This factoid brought to you courtesy a Star Trek fan who was given these scripts in the 1970s by someone who worked in the TV entertainment industry.
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