Reference for Yoma 82:4

אמר רב פפא כגון שלקח פרידה אחת אי עולה זבן מוסיף ומביא חובתו מדמי חטאתו והאי עולה אזלא לנדבה אי חטאת זבן אין מוסיף ומביא חובתו מדמי עולתו והאי חטאת אזלא למיתה

- R'Papa said: For instance, if he bought one single pigeon. If he bought it as the burnt-offering, then he adds to the money for his sin-offering the money for his [new] obligatory sacrifice, the burnt-offering [of the bird] becoming a freewill-offering; if he bought it as the sin-offering he may not add to the money for the burnt-offering for th purchase of his [new] obligatory sacrifice and that sin-offering is left to perish.

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