is a employer allowed to have a program that gives the rights to wear jeans on friday only to those who donate 2.50 a week to united way ?

Yes. As a general matter, an employer may impose any terms and conditions of employment it wants. No law prohibits enforcement of a dress code as to some employees and non-enforcement as to others, provided that the basis of discrimination isn't a prohibited one (such as race, religion, national origin, etc.).