as salam alaikum
there is no relation between a Sayyid girl marrying a non-Sayyid man and the sadaqah that is not allowed on Sayyids. So it doesn't make any difference that a Sayyid girl is from the progeny of Muhammad, peace be upon him and his family, in this regard. In Islam, Muslims are encouraged to marry a pious and religious people, committed to the Din in order to please Allah Almighty, be they Sayyids or not.
With prayers for your success.