What should one do if they vowed to slaughter a sheep on a specific day and the designated sheep died before that day?

If a person vows to sacrifice an unspecific sheep on a specific day, then designates it, and the sheep dies before the sacrifice, he is obligated to sacrifice a replacement sheep. However, if he vows to sacrifice a specific sheep, and it dies or becomes defective without any negligence on his part, then he is not obligated to provide a replacement, unless he was negligent, in which case a replacement is required, or an expiation for a broken oath according to Imam Ahmad. In the case of the question, it appears that the obligation has not been fulfilled, and he is obligated to sacrifice another sheep.