Commentary for Kiddushin 119:12

In such a case the woman is immediately betrothed and the man must thereafter give her two hundred zuz. If he does not give her two hundred zuz, the betrothal becomes invalid. The problem is that since he didn’t set a time limit he has an unlimited time to give her the two hundred zuz. Potentially, the only way for the betrothal to become invalid would be for him to die before he gives her the money. In such a case she would not be considered his widow and she would not be liable for yibbum. Therefore, this is not a particularly good way of performing betrothal, especially for the woman.

In such a case the woman is immediately betrothed and the man must thereafter give her two hundred zuz. If he does not give her two hundred zuz, the betrothal becomes invalid. The problem is that since he didn’t set a time limit he has an unlimited time to give her the two hundred zuz. Potentially, the only way for the betrothal to become invalid would be for him to die before he gives her the money. In such a case she would not be considered his widow and she would not be liable for yibbum. Therefore, this is not a particularly good way of performing betrothal, especially for the woman.