Can i make up an unpaid 8hr FMLA day in the same week on sat or sun to have my 40hrs.

I have a son with cancer he goes for treatment every tuesday for roughly 6-8 hours at times other appointment or treatment take place changing this in to an all day event. My manager has change, i have a new manager thats change everything in other words said i can not make up my hours on sat or sun at all use your vac or sick time if you want 40hr week.I think that this is a violation within 40hrs guarauntee within the UNION agreement.

Also i'm guaraunteed 40hrs within a week starting on first day (Monday) ending (Sunday). FMLA is an unpaid day UNLESS I USE VAC OR SICK TIME i never had any problems working on a Sat or Sun to make up Tuesday (FMLA) day with my previous manager.