Dear Chitrappa,
I would like to share you some of the points here below:-
1. Generally it is a mutual agreement between the company & the respective employees. {if any employees wants the salary advance, he/she has to submit the request letter to HR, the letter stating that the reason for salary advance, if the reason is genuine, the departmental head have to signed in that request letter & send the same to HR for approval}.
2. The amount of calculation for salary advance, based on his/her numbers of days he/she worked in that month, while the time of asking salary advance & amount of his full & final as on date of request letter submitted by the employee.
3. Once the approval given by the HR, the mode of repayment may furnished in that request letter itself, {it may furnish in one installment or more than one installments as per the employee's option & management policy}
4.In that request letter itself employees have to furnish he will be continually working with the company till the completion/refund/deduction of his/her salary advance, if any situation arise to resign his/her job, whether he has to refund the salary advance or the salary advance may adjusted against his full & final settlement.
5. Lets others/seniors may add more points on your above subject.