If you are on Family Medical Leave based on the Family Medical Leave Act, you are allowed up to 12 weeks of unpaid time off. You can use your paid leave time with it, so take the 12 weeks and let's say you have sick time and annual time for 3 of those weeks (at full salary). You would then only have unpaid leave for 9 weeks. If you are on FMLA for an injury, then your boss cannot require you to work and should not because it would disrupt your FMLA time off. Further, upon your return, your boss cannot simply lower your salary out of retaliation due to being off on FMLA. Contact your state labor department and Equal Opportunity Commission and give all of the details of what your boss is attempting to do (keep to the facts) and see if there are state and federal law violations for this attempt.