Commentaire sur Parah 7:4

קדש לי ואקדש לך – as for example, that each one filled water for himself, and this one came and mixed [with ashes] the water of that one, and that one mixed the water [with ashes] of this one, the one who mixed [the water with ashes] first what he mixed for his fellow is kosher/fit, for his fellow did not do any work other than until after his waters were mixed [with ashes], but when they (i.e., the waters) were mixed with ashes, no work/labor invalidates it. But what the second [person] who mixed [ashes in the water] for the first [person] is invalid, for the first invalidated in that he mixed [water and ashes] in the water of the second ‘person] before his waters were mixed [with ashes].

Introduction Beware! This mishnah might be a bit confusing, but it should all work out in the end. A classic example of rabbinic conundrums!

מלא לי ואמלא לך – the latter drawing of water/filling is kosher/fit, but the first drawing of water/filling is invalid, that is, that the waters that were filled were invalidated through the work of filling/drawing that he filled for his fellow.

[If one said to another] "Prepare the mixture for me and I will prepare the one for you," the first is valid. Two people drew water to use it for that hatat. Then one said to the other, "You mix the ashes for me and I will mix for you." The first mixture is valid but the second mixture is not because the second person was occupied with mixing the first person's ash and water between his filling his water and its being mixed with ash.

קדש לי ואמלא לך – that he had the waters filled/drawn and said to his fellow, “mix for me my waters [with ashes] and afterwards, I will draw [water] for you.”

[If he said,] "Fill the water for me and I will fill the water for you," that of the latter is valid. The second filling is valid, but the first filling is invalid for the owner of the water was occupied with filling the other person's water between his own water being filled and their mixing.

שניהם כשרים – for the work/labor does not invalidate after the mixing [of ashes with water]. And this also when the waters were drawn/filled for him, his waters are fit/kosher, that from when the waters were filled/drawn for him, he did not do any work.

[If he said,] "Prepare the mixture for me and I will draw the water for you," both mixtures are valid. The first mixture is valid for the owner didn't draw the water for the second person until his mixture was already prepared. And the drawing of water for the second guy only began after the first mixture was already prepared, so it too is valid.

מלא לי ואקדש לך – that the other that had waters filled/drawn and he said to him: “Fill for me [waters] first, and afterwards I will mix [the ashes in the water] for you.”

[If he said,] "Fill the water for me and I will prepare the mixture for you’, both mixtures are invalid. In this case both are invalid. The first mixture is invalid for the owner of the first water prepared the second person's mixture between the filling of his own water and their mixture. The second mixture is invalid for the second person was occupied with drawing water for the first person between the drawing of his own water and their mixture.

שניהם פסולים – this one who filled until his waters were not mixed [with ashes], and this one who mixed [ashes with water] for his fellow from when his waters were filled up/drawn.