Are both sentences OK?
I'm wondering if it's okay to get an advance on my salary.
I was wondering if it was possible to get an advance on my salary.

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Answer

Both sentences are correct, but the second one would make more sense to ask to your superior. Usually this sort of question requires a little more formal language, so the second sentence would work better.

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Second answer is better. BUT. It's "I was wondering if it IS okay to get an advance on my salary."

That's all.

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I agree with Tom.  Both sentences are correct, but the second one is much more polite.