How do we legally dock an exempt worker’s PTO leave?

Q: Moving into the new year, the owners of my company would like to make a few of our employees salaried. They all meet the "test." The owners would like to offer PTO time (3 weeks vacation and 3 personal days). Their thought is to dock pay for each day the salaried employee takes PTO beyond their allotted time. The owners have agreed between themselves to do this, since one takes more time off than others.  Is this legal in the state of California to do with salaried employees? – Fonda, California