Is it permissible to sell the ram designated for the wedding feast or to العشر (slaughter it according to Islamic law), and replace it with a smaller one, or with a male goat, especially since it is growing larger and losing meat, and the wedding has been delayed?

There is no blame upon you for disposing of this buffalo, whether by selling it or otherwise, and you are not obliged to replace it with another, as there was no explicit verbal commitment to sacrifice it. Therefore, it is not a vowed offering (manthoorah), and you may dispose of it as you wish. The mother's statement, "We will do such and such with it," is merely a promise and does not carry any legal implication. This is assuming the mother was the owner of the buffalo; otherwise, her statement holds no weight, because a vow is only valid when made by the owner of the vowed item.