Nedarim, Chapter 3, Mishnah 5. This mishnah deals with a vow whose subject is not forbidding an object to a person, but rather dedicating the object to hekdesh - that it should belong to the Beis HaMikdash. As we will see, this is proven from the precise wording of the mishnah.
The two cases in the mishnah:
"Harei netios ha'elu korban im einan niktzatzos" - a person who has saplings, and he sees that the strong wind is breaking them and is afraid they will be uprooted, says: if they are not uprooted and not cut down by the wind, these saplings shall be a korban, meaning hekdesh.
"Talis zo korban im einah nisrefes" - a cloak is lying near a fire, and he is afraid it will burn, so he says: if it is saved from the fire, it shall be hekdesh.
In both cases the mishnah rules that "yesh lahen pidyon" - the hekdesh takes effect, and they can be removed from the status of hekdesh, but only through redemption.
The precision in the word "korban":
This is a case of hekdesh and not of a prohibition of benefit, since the one taking the vow said that they are a korban, and not "harei elu alai k'korban" (these are upon me like a korban). Had he said "upon me," that would be merely a prohibition of benefit that he takes upon himself; from the fact that he said that they are a korban, it is proven that his intent is that they become hekdesh - except that he made it conditional, only if they are not uprooted and not burned.
Why the hekdesh takes effect:
The point the mishnah is making is that the hekdesh actually takes effect. We do not say that in his heart he never intended them to become hekdesh, because he was certain they would not be saved - that the cloak would burn and the saplings would be uprooted - and therefore his declaration was not made with full intent. Such thoughts are "devarim shebalev" (matters of the heart), and matters of the heart are not binding, since they are not evident from his words. Therefore we establish that his words stand as they sound: if they are saved, his wish is that they be hekdesh - and they indeed are hekdesh, to the point that they require redemption.
"Ad sheyikatzetzu" - a formulation that has no redemption:
In contrast, one who says "Harei netios ha'elu korban ad sheyikatzetzu" (these saplings are a korban until they are cut down), or "Talis zo korban ad shetisaref" (this cloak is a korban until it burns) - this wording indicates that "ein lahen pidyon" - they have no redemption. More precisely: even if he redeems them, they immediately return to being hekdesh, since his wording was that they remain a korban until they are cut down or until they burn, which means that in any case, even after redemption, they revert to their sanctity.
Nevertheless, the redemption itself does take effect: the money with which he redeemed them does indeed become sanctified. It is only that immediately afterward the objects become sanctified once again, and so it is with each and every redemption - the redemption takes effect with regard to sanctifying the money, while the objects return to their sanctity.
All of this applies as long as the saplings have not been cut down and the cloak has not burned; once they have been cut down or burned, they are no longer hekdesh.
Two points raised by the Gemara:
The Gemara records a dispute regarding the situation after they have been cut down or burned: whether they need to be redeemed once beforehand, or perhaps once they have been cut down there is no longer any need for redemption and they are permitted immediately.
The rule that the object becomes sanctified again with each and every redemption until it is cut down applies specifically when the one who dedicated it is the one redeeming it. But if another person redeemed it, the redemption takes effect and the object does not become sanctified again.