1
I want to know if it’s possible that I leave a bit earlier today.
I want to know if it’s possible for me to leave a bit earlier today.
When we mean that a person can do something, use possible for + person + to-infinitive: ‘Is it possible for us to change the date?’
Use possible that + clause to say that something may happen: ‘It’s possible that the train will be late.’
2
I’d like to get my money back today if is possible.
I’d like to get my money back today if possible.
I’d like to get my money back today if it’s possible.
Both if possible and if it’s possible are correct. Do not say if is possible.
The shorter expression if possible can go at the beginning or end of a sentence: ‘If possible, please reply today.’ ‘Please reply today if possible.’