TDS-u/s 194A

my client has interest income and he dont want his income cut with tds and expects, his income would be much lower than the basic exemption limit fixed by the income tax. Now my question here is how would one know his income would be less than basic exemption at the begining of year or any point of time in a year  and say if his income exceeded the basic exemption limit after submitting the Form 15G/15H(declaration asking for not to cut tds) to a banker.  how can we treat this type situation pls explain clearly. thanks for all the members.