3,008. You Can't Consecrate a Future Item, But....
Hilchos Arachin Vacharamin 6:31
The Rambam opines that even though one can’t consecrate something that has not yet come into existence, if he takes a vow to consecrate such an item, he must consecrate it when it comes into existence. Failure to do so violates the prohibitions against delaying to pay (Deut. 23:22) and desecrating one’s word (Num. 30:3); it also neglects the obligation to act according to the statements of one’s mouth (ibid.), the same as is the case with all other vows.
Hilchos Arachin Vacharamin 6:32
Pursuant to the previous halacha, if someone vows to consecrate whatever his net will bring up from the sea, or to give to the poor the fruit that his field will produce, or to proscribe all of his annual income, or to use it to redeem captives, or any comparable statement, he must act according to his words when he acquires the thing in question. This is because all such things are vows, not consecrations.