I used to work as a Deliver Driver/Installer for an AV company.
Many times I had to do over time to complete jobs. As a verbal agreement with my boss she said “We don't pay overtime but you can have time in lieu and use it whenever you want as long there are no jobs on”. I agree, I didn't have a problem with it.
About 3-4 months later, I had put in place for me to have 3 days over (We'd, Thur and Fri) because I had roughly 30 plus hours of time i lieu and there was no work.
When the boss found out she immediacy pulled me in her office and said I couldn't have the time off. I said “well it's my time In lieu”.
She turned and said “You don't get to choose when you take the time off”. I replied “Fine, I won't take the time off. But I'm future I will no longer be doing over time. If I'm in the middle of a job and it's 5:30, I pick up my tools and leave.”
She wanted a bit of time to think about it. She eventually “gave me permission” for the 3 days off.
I left not long after that because she stopped the time in lieu system.