We indeed find that a woman can appoint a shaliach to accept her get from her husband or from his shaliach. Rav Baruch Ber Leibowitz explains that although she is not the ba’alat hamamon (she does not own the get) she can appoint a shaliach because the type of shaliach that she appoints is of a higher level of shlichus. Her shaliach is in her place; his hand is like her hand. For such a level of shlichus one can appoint a shaliach even if he is not the ba’al hamamon. Based on this we can suggest that when Hashem told Moshe to become a shaliach for Aharon and his sons and accept the redemption money, Hashem made Moshe the highest-level shaliach, which can work even when he is not the ba’al hamamon.
We can now answer that there is no question on the Rivash and the Rema because they only prohibit the father – not the kohen – from appointing a shaliach.
