2,788. Adding a Condition to a Vow
Hilchos Nedarim 7:17
Let’s say that a man’s son-in-law is prohibited to benefit from him, and the man wants to give his daughter money for her benefit. He gives it to her saying that the money is a gift on the condition that her husband has no authority over it; rather, it is only to be used for what she eats, wears, etc. Even if he says, “…it is to be used for whatever you want,” her husband doesn’t acquire it and she may use it as she sees fit. However, if he only says, “...on the condition that your husband has no authority over it” without specifying a purpose for the gift, or even that it’s for whatever she wants, then the husband acquires it for purposes of deriving benefit. Such a gift is therefore prohibited because he’s not allowed to benefit from his father-in-law.
Hilchos Nedarim 8:1
If someone makes a vow or an oath and at that time he adds a condition to the vow, it’s the same as making the vow or the oath dependent on that condition. If the condition of the oath isn’t met, whatever it was that might have been prohibited remains permitted.