暇（なら・としたら）ちょっと手伝って下さい。
Hima (nara/toshitara) chotto tetsudatte kudasai.
Both the options seem grammatically correct.
Is it not so?
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暇としたら is not grammatically correct. と here is related to the quoting particle and for quoting a sentence, you want a full sentence, here 暇だ hima da, giving 暇だとしたら hima da to shitara. In any case, I think the next two options are better choices of saying "if you have time / if you're free"
暇なら、ちょっと手伝ってください。
hima nara, chotto tetsudatte kudasaiwhich is correct just the way you said it.
暇だったら、ちょっと手伝ってください。
hima dattara, chotto tetsudatte kudasai暇 でしたら、ちょっと手伝ってください。
hima deshitara, chotto tetsudatte kudasai (polite)
I myself would probably use the second one, although choosing between them would depend on the situation.
We have a question explaining the differences between なら, ～たら and ～えば: Differences among -たら、なら、-んだったら、-えば, etc
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I disagree that と is the quotation particle here; it's the "as"/supposition particle. – istrasci Oct 1 '14 at 4:05
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@istrasci Hmm... I only know one name for と when it works like this grammatically. It's called "as"/supposition particle? – Earthliŋ♦ Oct 1 '14 at 9:53
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@Earthlin: the "as" can be seen often in the form 〜として (「〜の資格で」; see examples in definition #1 here). The supposition/assumption case can be seen in ex. 815 here. – istrasci Oct 1 '14 at 15:36
