Income tax deduction in operation and maintenance contract

Please advice that how much tax should be deducted in the bills of Operation and Maintenance Contract of Airconditioners / lab facilities run by the OEM or other venders. Duration of the contract is one year and sometimes more than one year. However, payment is made on six months basis. 

We don't know the exact imcome of the contractor/vender. Is it safe to deduct tax even if the value of the bill/bills is less than 30,000/-

Thanks & Regards