1. Sir, contribution under ESI Act, 1948 is payable on the "wages" paid or payable and not on the basis of rate of salary or wages. However, wages paid on account of sanction of leaves with wages are also wages on which contribution is payable under said Act.
2. If the amount of wages in any month is only Rs.8000, then contribution, in my opinion, is required to be deducted on the said amount of wages you received.
3. In case, the employer is deducting contribution on higher side from your wages, then you can take up the matter firstly with the employer, and then in case of no response, with ESIC.