Can an employer flex an employee out for hours not yet worked in a specific work week in order to avoid overtime from happening.

We receive our schedules a week in advance. According to our schedule/hours that we will be working the following week, we will have some overtime from opening or staying late/closing. My employer will flex us out, have us leave early say on a Monday for overtime hours that they are expecting we will receive throughout the week. If for some reason we do not end up having overtime, then if we do not meet our 40 hours because we were flexed out earlier in the week, they take the time from our PTO bank. I don't understand how they can flex us for hours we haven't even worked yet. Can they?