Clubs may include an option to their dues billings to members for giving to The Rotary Foundation to help encourage their membership to participate. However, donations to The Rotary Foundation are not to be part of the regular, mandatory dues payments.
The Rotary Code of Policies addresses this:
5.020.2. Prohibition of Mandatory Contributions to The Rotary Foundation 
The Rotary Foundation has developed on the basis of voluntary contributions. Contributions to the Foundation shall not be a condition of membership, and any reference implying such condition of membership shall not appear on the membership application card. Clubs are prohibited from enacting a bylaw that makes contributions to the Foundation a condition of membership. Any reference to such contributions on membership identification cards is not authorized. (November 2004 Mtg., Bd. Dec. 59) 
Source: November 1964 Mtg., Bd. Dec. 192; June 1999 Mtg., Bd. Dec. 298; Amended by November 2004 Mtg., Bd. Dec. 59
Please let me know if you have any other questions!