A lawyer once explained to me that, because an exempt salaried worker may be expected to work more than 40 hrs without additional compensation, the employer can expect a worker to work around 40 hrs/wk, but cannot require specific number of hrs, as long as that person is performing a reasonable amount of work for the workweek--meaning sometimes it might be more, other times less, but overall it will probably average 40+ hrs.
The lawyer clarified that this is the tradeoff an employer makes for paying a salary rather than an hourly wage. Because if the employer can impose a minimum of 40 hours, then a salary is never advantageous over an hourly wage for a worker.
Can anyone substantiate this claim that an employer cannot require a minimum number of hours? What is the specific passage in the FLSA (Fair Labor Standards Act) or other US or state legislation that backs it up?
If it makes any difference, this lawyer is from Iowa, but if I recall correctly, the claim may not be limited to Iowa.