Dear Ashoke,
With regard to to your first query , My answer is yes , you can give salary on target base to Sales employee but ensure that it cannot be under any circumstances less then applicable Minimum Wages in your state.
My answer to the your second query, yes you can mention Petrol Allowance (rather than conveyance allowance) and Mobile Allowance in Salary Slip but all these Allowances are subject to statuary deduction like ESI , if gross salary is within Rs.15000 per month.Besides, deduction of Professional Tax & IT is also applicable in accordance with law.
Instead of Petrol Allowance if you pay him as Conveyance Allowance the concerned employee through salary slip , he/she will get IT relief @Rs 800/ per month on which no IT is payable .
Alternatively, you may consider to pay him reimbursement of petrol and Mobile expenses by way of payment through voucher in cash in order to avoid statuary deduction from employee's salary for the benefit of the employee concerned. Of course, it depends on your company Policy.
Feel free to contact me in case of any further clarifications.
Thanks and regards,
Sibabrata Majumdar
Management Consultant -Legal & HR
Kolkata
(M): 9830023706