Commentaire sur Parah 7:3

אם לו פסול - if the mixing [of ashes and water] is his, it is invalid, for perhaps, the labor/work preceded the mixing [of the ashes and water] and the water was invalidated.

Introduction Today's mishnah describes situations where a person prepares the mixture of hatat ash and water and at the same time performs another act, and yet the mixture is still valid.

ואם לאחר – is the mixing [of ashes and water].

One who prepared the mixture with one hand and did some other work with the other hand, the mixture is invalid if he prepared it for himself, but if he prepared it for another man, it is valid. Performing work while mixing the ash and water doesn't disqualify the mixture if the preparation is being made for another person, as we saw in mishnah one. However, if he makes it for himself, it is invalid.

כשר – for the owner of the water did not perform any labor/work.

If he prepared a mixture both for himself and for another man, his is invalid and that of the other man is valid. The same principle is operative here he invalidates his own mixture because he is engaged in some other work, but he doesn't invalidate that of the other person.

המקדש לו ולאחר – both of them are as one. Alternatively, he mixed [ashes and water] for his fellow prior to his own.

If he prepares mixtures for two men simultaneously, both are valid. Since none of the preparations were being made for himself, both are valid.

שלו פסול – for the mixing [of ashes and water] of his fellow is considered work to invalidate his waters.

שניהם כשרים – for the water is not his that it can be invalidated through him.