Any donation receipt would have to come directly from the 501(c)(3) organization. Since the exempt organization is not taking control of the $20 cash given, it will not be able to issue a receipt for that amount. However, it may be willing to issue a thank you letter to the directors of the church board that contributed food towards the fundraising event. The individual directors have to be able to convince the IRS that their noncash contributions of food were for the benefit of the exempt organization and should be treated as noncash contributions to the organization.
For more information on charitable contributions, see Richard Hammar’s annual Church & Clergy Tax Guide.