The standard ketuba that is written at weddings includes a guarantee given by the husband that all of his possessions are to be made available to pay the ketuba in the event that he dies or they get divorced. Clearly, throughout their marriage, the husband can buy and sell his possessions since no single piece of property is set aside to pay the ketuba.
We have already learned that should a man die, leaving a widow and no children, the widow becomes a yevama, and one of the brothers of the dead man has the opportunity of fulfilling the mitzva of yibum. In such a case, the original ketuba remains in force, and the yavam inherits the dead man’s possessions, all of which remain obliged to the ketuba. That is to say, the new husband cannot sell any of his late brother’s possessions, since they are all needed to guarantee payment of the ketuba at some point in the future.
Rashi discusses at some length why there are differences between the original marriage, where the written guarantee suffices, and the levirate marriage, where there is a need for all of the property to remain available at all times to pay ketuba. His central argument is that the yevama does not have the same level of faith in her second husband – who did not personally guarantee payment of the ketuba – as she did in her first husband, who took personal responsibility. Many rishonim follow Rashi’s lead, to the extent that the Meiri writes that it was common practice for the yavam to write a new ketuba in which he guarantees payment in the event of death or divorce. The Ritva adds that we also need take into account that the original ketuba was likely a 200 zuz ketuba, as it was written for a woman who had never before been married, while the yavam’s ketuba would only be for 100 zuz.