At the end of the day, margin balance will be recalculated. In case of a shortfall, it will be debited to the account balance and in case of a surplus, it will be credited to account balance. If account balance is not adequate to meet margin requirements, the user will not be allowed to execute transactions that will result in increase in positions. He or she however can reduce the positions and after reducing, release the margin money and create fresh positions for futures, for any of 3 months. But after every squaring up of a transaction, partly of fully, profit or loss for the position squared up is calculated and adjusted in the account balance.
Account balance is calculated after taking into consideration the profit or loss.
Whenever the margin money has to be calculated, it is on the net account balance. At the end of the month i.e. on the last day of a calendar month, one month's futures will be settled on the basis of the closing prices. When one wants to buy, he can do so at offer rate and if one wants to sell he can do so at bid rate. There will always be difference in bid and offer price. In other words even if there is no movement in market, if one buys and sells will incur loss equal to difference in offer and bid rate which is analogous to jobbing spread.