Mishnayos Temurah Perek 4 Mishnah 2

In the case of one who designates a sin offering, and the animal was lost, and he designated another in its stead and sacrificed it, and thereafter the first animal was found; that is a sin offering whose owner achieved atonement with another animal, and it shall be left to die. In the case of one who designates money for purchase of his sin offering, and the money was lost,and he designated an animal as a sin offering in its stead, and he sacrificed it, and thereafter, the money was found, it is prohibited to derive benefit from the money, as the money attains the halakhic status of the sin offering that was to be purchased with it, and that sin offering would be left to die because the owner achieved atonement with another animal. Therefore, he must take the money and cast it into the Dead Sea, from where it cannot be recovered.

הַמַּפְרִישׁ חַטָּאתוֹ וְאָבְדָה, וְהִקְרִיב אַחֶרֶת תַּחְתֶּיהָ, וְאַחַר כָּךְ נִמְצֵאת הָרִאשׁוֹנָה, תָּמוּת. הַמַּפְרִישׁ מָעוֹת לְחַטָּאת וְאָבְדוּ, וְהִקְרִיב חַטָּאת תַּחְתֵּיהֶן, וְאַחַר כָּךְ נִמְצְאוּ הַמָּעוֹת, יֵלְכוּ לְיַם הַמֶּלַח: