Can you guys log on to a mainframe and trade in and out of open time?

If your line is 72 hours, is it possilbe to trade it down to lets say 45, understanding that you lose the diff in pay of course. (I'm thinking this can happen but realistically doesn't because of staffing)

Can you advertise trips on a mainframe and let someone else pick it up, (of course once again not getting the pay)?