Before, when our grandfathers and grandmothers were young (and before), there was a custom for the bride to bring something material to the family she's marrying into. In Croatia it's called "miraz", and in English, the term is translated into "dowry". Also, it was the custom for the groom to "buy out" the bride from her family, and the bargaining was made when the groom would come to get the bride before they go to church together. Usually, the bride's family would try to get as much as possible, so the joke was to bring him the false bride because, that's the bride he can get for the offered value. Of course, after some time, the groom would offer a bit more and the family would bring the real bride and then they could go to church and get married.
Nowadays, the bargaining and the dowry part are not common anymore, but the false bride is still a very popular custom. :)