Hi Puspa,
According to the normal process, For fixed allowances will based on the effective date according to your information submitted.
Example : in payroll August 2015 as pic below,
Our system will paid his for title allowance in August 2015 = 16000 because title allowance in our system haven't set the pay code for paid back pay.
From the above information will affect is tax August of employees will be higher than normal.
But the next month tax will back to normal.