So, this just happened to my wife. Her colleague called in sick who as supposed to be oncall and will need to do a night shift. My wife agreed to cover her, since it'll be considered overtime and wouldn't need to do a day shift the following day, naturally.
Prior to the night shift, she finished work at 4pm, and was told to start her night shift at 11pm. Just before 11pm, she was told to start at 1am instead, ok no big deal.
She started her shift as told at 1am. At 6:30am, her manager called her, saying that as she had at least “8 hour rest between shifts” prior to starting her night shift, her night shift will be considered a normal rate shift without any penalties, and that she “shouldn't rush to finish” in the morning and should continue working a full shift until late morning/mid day (her day shift is typically 10 hours), otherwise it'll be considered taking a sick leave if she was to leave early at 8am (when everyone else starts and night shift people can handover) for the rest of the day (essentially wasting a sick leave for what should've been a day off). She ended up finishing work at 2pm getting paid normal rate.
Apparently this has happened to others as well. I was so mad I wanted to call her manager but was stopped by my wife. She said she'll just be careful next time when picking up extra shifts. F*ck.
TLDR: my wife volunteered to cover a night shift (overtime pay) due to sick leave but was paid a normal rate in the end.