The Torah gives clear parameters for the animal that is to be brought as the korban Pesah. It must be a male that is one year old (see Shmot 12:5). What if an animal is set aside as a korban Pesah and it does not meet those basic criteria?
Mishna: In the case of one who separates a female animal for his Paschal lamb although the Torah requires a male, or a male that is in its second year although a Paschal lamb must be an animal that is in its first year, the animal is left to graze until it develops a blemish and becomes unfit, and it is then sold and its money is used for free-will offerings or peace-offerings.
What is left unclear in the Mishna is what is to be done with the proceeds. The Mishna appears to offer two contradictory rulings. According to the standard text the money should be used for a nedava (a voluntary offering), a Shelamim. Actually there are variant readings of the Mishna. The Jerusalem Talmud reads that the money should be used as a nedava. Many other sources say that the money should be used for a Shelamim, the standard use of a korban Pesah that was not sacrificed.
In his commentary to the Mishna, the Rambam explains – like the Yerushalmi – that a korban nedeva should be brought with the money. According to Rashi’s reading of the Mishna, the money should be used for a Shelamim. The Rambam in his Mishneh Torah agrees with that ruling, but only under certain circumstances. According to the Rambam, once we realize that this animal cannot be brought as a korban Pesah, we set it out to pasture, hoping that it will develop a mum – a physical blemish that will make it unfit for sacrifice. At that time it can be sold and with the proceeds an animal appropriate for a korban Pesah can be brought. If, however, the animal does not develop a blemish until after that time, a different animal will have to be purchased with other monies, and when this animal develops a blemish a korban Shelamim will be bought with the proceeds.