I'm fine for time isn't appropriate answer. They can say we're free.

Generally speaking, when someone uses that form - ''fine/OK on/with/for time', it's meant in a way that implies you have enough time to complete a task before a deadline or before another task is scheduled. The report isn't due until next month, so we're fine on time. My plane leaves at 4:00, but this should only take 15 minutes, so I'll still be fine on time. I can't ever think of it being used to indicate that one is not busy per se, however, I suppose if someone asked if you 'had time' to do something, knowing you had other tasks or a schedule, and you had plenty of time to do said task it could work. A: I know you're leaving in a couple of hours and I don't want to make you late, but do you think you'll have time to go over those reports with me before you leave? B: Absolutely! I'm actually almost done here, so I'm fine on time. Cheers!