With regard to your first query, my answer is yes, you can give a salary on a target basis to sales employees, but ensure that it cannot be under any circumstances less than the applicable Minimum Wages in your state.
Regarding your second query, yes, you can mention Petrol Allowance (rather than conveyance allowance) and Mobile Allowance on the salary slip, but all these allowances are subject to statutory deductions like ESI if the gross salary is within Rs. 15,000 per month. Additionally, deductions of Professional Tax and IT are also applicable in accordance with the law.
If you pay the employee as Conveyance Allowance instead of Petrol Allowance through the salary slip, the concerned employee will receive IT relief at Rs. 800 per month on which no IT is payable.
Alternatively, you may consider reimbursing petrol and mobile expenses by providing payment through vouchers in cash to avoid statutory deductions from the employee's salary for the benefit of the concerned employee. This, of course, depends on your company policy.
Feel free to contact me for any further clarifications.
Thanks and regards,
Sibabrata Majumdar
Management Consultant - Legal & HR
Kolkata
(M): [Phone Number Removed For Privacy Reasons]