No balance transfer is guaranteed to be free, but some credit cards do offer the potential for a free balance transfer from time to time. More specifically, it’s not unheard of for a credit card to offer a 0% intro APR on transfers without charging a balance transfer fee or an annual fee. So if you get such a card and repay what you owe by the time regular rates take effect, you’ll have yourself a free transfer. But if you have a balance remaining at that point, or you miss a monthly minimum payment along the way, you’ll end up paying interest.
Free balance transfer credit cards are not common, though. Chase Slate® is a notable example. But there used to be far more options. Now, banks tend to offer either no balance transfer fee credit cards or 0% balance transfers, but not both.