2,756. Vows Taken to Encourage Others
Hilchos Nedarim 4:3
Similar to the cases in the previous halacha, vows taken strictly to encourage others remain permitted. For example, let's say that one person imposed a vow on another to eat at his home and the other took a vow not to eat there because he didn’t want to trouble his friend. In such a case, regardless of whether he ultimately eats there or not, both of them are exempt. Similarly, let's say that a salesman takes a vow not to sell an object for under a sela and his customer takes a vow not to buy it for more than a shekel. If they compromise at three dinar, both are exempt. The same is true in all similar cases because neither one means it completely. Rather, the vow is only intended to encourage the other person without making a firm commitment.
Hilchos Nedarim 4:4
The preceding halachos discussed four types of vows whose objects remain permitted: those made under coercion, unintentionally, employing exaggeration and to encourage others. However, one may not take such a vow intending to annul it as per Numbers 30:3, “He shall not desecrate his word.” We see from this that one should not make his words trivial.