Have you ever heard of paying back salary for not working a certain amount of hours?

While hardly common, this is legal if--
1) The employee is aware of this term or condition of employment before he or she begins working that year, so that by working, he or she can be said to have agreed to these conditions of employment; people have the legal right to agree to many variant working/compensation arrangements so long as 2) and 3) below are also met.
2) The employee earns at least minimum wage, no matter what.
3) There is no illegal discrimination going on--e.g. it is not only minority, or only female, etc. employees who get this payment scheme.
HOWEVER, even if legal, this can change a salaried employee to being effectively an hourly employee and therefore eliminate what would otherwise be his or her exemption from overtime. Salaried employees are paid the same no matter how many or few hours they work--they can be terminated for not working as much or as hard as their employer likes, but their salary cannot be based on hours worked. If their time worked is tracked for pay purposes and their pay is reduced if they don't hit a certain number of hours, an argument can be made that this effectively results in them being paid as hourly (time based) employees, which could mean that in any week that they work more than 40 hours, they are owed overtime.