Question: When people say, as is the custom, that "This fowl is for the Sabbath", or for the Festival and the like – Is this to be considered a vow? And if it is considered a vow, can one leave it aside and purchase some meat or another fowl in its place?
Reply: It seems to me that this counts completely like a vow, since the pleasure of Sabbath and Festivals is a commandment. Why would one say that it is not a vow? As for the question of replacing it, it seems that one may do so, see Shulchan Aruch Yoreh De'ah on laws governing charity, Section 259, a: A person who designates a stone [amount] for charity or states that "this stone is my gift" and puts it aside may change it before he comes to the beadle, borrow it for themselves, or lend it to a fellow person and pay with a replacement. The sanctity of the pleasure of Sabbath is certainly no greater than the sanctity of coins for charity, and since we find that one can replace funds designated for charity, that is the law concerning what has been designated for the pleasure of the Sabbath. One can replace it and eat something else in its place, and certainly if it is of better quality than is the first, and provides more pleasure than the one originally designated for the Sabbath.