It would be bad form to do this, but as a former Personnel Manager I wouldn't bother to sue you - just not worth the effort.
It is actually skin off an employer's nose to let someone have unpaid sabbatical - one has to advertise the position, recruit, train etc. A sabbatical is basically an agreement on both sides to suspend the contract for an agreed period. Technically you would have to give your notice period at the end of the sabbatical and work out your notice.
A paid sabbatical would normally be mentioned within the employment contract - in the Company handbook, polices or wherever. It is a significant employee benefit, and some companies offer them as a reward for long service (IBM, for one, used to do it).