It seems to me that this is obvious and it is unneccesary for the Gemara to say it. This is because the reason he is exempt if he sells the animal after he makdish it, is not only because he sold hekdesh's animal, but for a much more basic reason - his sale simply did not take effect. This is because one cannot obviously cannot sell an item that belongs to someone else. For instance if I try to sell your house to someone else this obviously has no validity because I do not own your house. In the same way, once he makdish the animal, it belongs to the Beis Hamikdash, and it is meaningless if afterwards he tries to sell it, so it is superfluous to tell us that he is exempt from the 4 and 5 payment.

In contrast, there is a chidush that when he slaughtered the animal belonging to hekdesh that he is exempt, because the physical action that he performed by killing it clearly took effect. The chidush is that nevertheless he is exempt from paying 2 and 3 because one is only liable for the latter if one slaughters a private person's animal but not if one slaughters the Beis hamikdash's animal.

It is true that the Mishnah 74b stated that if he sells the animal after hekdesh that he is exempt from 4 and 5, but here since the mentioned tevichah it also mentioned mechirah even though there is no chidush with mechirah. However the gemara 76a could not have mentioned mechirah because it would not have been appropriate to say that the reason he is exempt is because he sold hekdesh's animal, because there is a simpler reason he is exempt - because the sale did not take effect, as we explained above.

Kol Tuv

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