If one says, “I promise to bring a flour offering on a griddle,” and brings instead an offering prepared in a deep pan, then instead of a promised hard loaf he brought a soft and spongy one. He has not fulfilled his obligation and has to bring another offering, but the one he brought is valid. We regard it as another free-will offering.
However, if he pointed at the flour and said, “This flour is designated to be brought as an offering on a griddle,” and instead he made one in a deep pan, his offering is completely invalid. Why is this difference? In the first case, he created a personal obligation, but he was free to bring another additional offering. In the second case of flour designation, the flour acquired the sanctity of a specific type of offering, and this cannot be changed.
If one promises a flour offering of barley, he nevertheless has to bring one of wheat. His starting words, “flour offering” are considered decisive, and the conclusion “of barley” - which is meaningless, because there are no personal barley offerings – is disregarded.
Art: Marianne Preindelsberger Stokes - Polishing Pans