3,009. Proof for the Rambam’s Opinion
Hilchos Arachin Vacharamin 6:33
Proof for the Rambam’s opinion (in the previous halacha, that one can make a vow about future items) can be seen in Yaakov’s statement in Genesis 28:22, “All that You will give me, I will tithe,” which 31:13 refers to as a vow. If someone says, “I won’t leave this world until I have become a nazir," he must observe nezirus even though he didn’t actually make a nazirite vow. Since he said that he would, he must observe the rules of a nazir. Our case is comparable, so such a ruling is appropriate.
Hilchos Arachin Vacharamin 6:34
A consecration made in error is ineffective. For example, if someone says that if a black ox leaves the building first, it will be consecrated, and a white ox comes out, it’s not consecrated. If he says that if a gold coin comes into his hand first, it’s consecrated, and a silver coin came first, it’s not consecrated. If he says that if a barrel of wine comes into his hand first, it’s consecrated, and a barrel of oil came first, it’s not consecrated. This is true regardless of whether wine or oil is more expensive in that locale. If he tries to extend the consecration to a second item, saying that item X is like item Y, then the second item is effectively consecrated. The same is true in all comparable cases.