Hilchos Ishus 6:2
The four criteria for conditional transactions are: (1) the condition must include both what will happen if it is met and if it is not met; (2) the ramifications of the condition being met must be stated before the ramifications of it not being met; (3) the condition must be mentioned before the document to which it refers is completed; and (4) the condition must not be something impossible. If one of these criteria was not met when an agreement was being made, then the condition is invalid, as if it never existed at all. In such a case, marriage or divorce would be implemented or a business transaction would be completed instantly, as is the case when no condition is made.
Hilchos Ishus 6:3
Based on the above parameters, let’s say that a man says to a woman, “If you give me 200 zuz, you are consecrated to me through this dinar but if you do not give it me, then you are not consecrated.” After making this condition, he gives her the dinar. This is a valid condition and marriage depends upon it being fulfilled. If she gives him 200 zuz, then marriage is effected; if she doesn’t, it is not.