Is it legal for a church to have a policy that demands or mandatorily requires employees give a portion of their income as a requisite for employment?
Yes, with some caveats.
In Corporation of the Presiding Bishop v. Amos, 483 U.S. 387 (1987), the United States Supreme Court approved the firing of a church employee who failed to maintain a temple recommendation for his local congregation of the Church of Jesus Christ of Latter-day Saints. The local temple could not recommend him for employment unless he was a member in good standing. To be in good standing, the local temple requires its members to prove that they have paid their tithes. Since the church employee failed to provide the temple recommendation, he was terminated.