Hillary Clinton and Senator Boxer should remind the parties involved of what happened to the last couple that got married at the White House.
More like “I am giving you this thing of value to show you that I am serious about my promise.” From there follows the popular belief/custom that if a man breaks the engagement, she keeps the ring. If she breaks it, she gives the ring back.
I don’t know about the cows, but the sheep and the goats could really help out on the national debt by reducing the cost of maintaining the White House grounds and possibly even the Washington monument too could use the sheep and the goats.
This will require further investigating though … I would be glad to request a $100,000 dollar grant to study the effects of sheep and goats working together on the same property. 
Well, there’s certainly precedent for it: http://www.whitehousehistory.org/history/white-house-facts-trivia/facts-wilson-sheep-world-war-one.html
It can be hell trying to wear a cow on your finger.