Tds on incentive paid

Dear All!

Please clarify me as regards the Following issue. 

One of our client has made provision for incentive to be paid to Key managerial personnel as on 31st March, 2012. 

They have not deducted TDS on the above amount as on that day and have subsequently deducted TDS when the payment was made on Aug 2012. My contention was there is a delay in deduction of TDS. 

Whereas our client argue saying that the provision was made for selected personnel and the amount payable to every individual was not ascertainable and therefore TDS was not deducted while making provision. Also, TDS in respect of incentive (salary) paid is deductible only at the time of making payment. Please clarify as to when is the right time at which TDS ought to have been deducted.