Boss took half week Work and Salary after returning after FMLA. Is that legal?

It is unlawful for any employer to interfere with or restrain or deny the exercise of any right provided under this law. Employers cannot use the taking of FMLA leave as a negative factor in employment actions, such as hiring, promotions or disciplinary actions; nor can FMLA leave be counted under "no fault" attendance policies. Under limited circumstances, an employer may deny reinstatement to work - but not the use of FMLA leave - to certain highly-paid, salaried ("key") employees. See: http://www.dol.gov/elaws/esa/fmla/faq.asp for all FAQs.

If your employer reduced your workday as a "disciplinary action" for taking FMLA, then technically this is unlawful, but undoubtedly hard to prove. I would suggest asking your boss why your working hours were reduced and, if it is only for the week, to make sure that after this week has passed, that you begin working your full 8 hours again.