What should a participant in a sacrificial cow do after the cancellation of the celebration for which some shares of the cow were intended to be used as a feast: should he sell the cow and donate a third of its price, or buy a sheep with the price of that third, or what should he do?

According to the Shafi'is and Hanbalis, the Udhiyah (definition) becomes obligatory through verbal declaration, not merely by intention. It becomes Udhiyah by intention only according to the Hanafis. As for the Malikis, it becomes obligatory through slaughter or vow. If you verbally declare that the cow is an Udhiyah, then slaughtering it becomes obligatory. However, if you only intended it, it is preferable to slaughter it to avoid differing opinions. It is permissible to sell it and buy a sheep that fulfills the requirement, and in this case, there is no obligation to spend one-third of its price.