Dear Rajarajan,
If you have crossed 10L limit and have to pay service tax, it is possible that you will have to pay service tax at the time your receipt of the money or money worth occurs. If the company has given you the voucher, you are still a creditor to the company and therefore the credit cannot be treated as a receipt. Service tax is on receipt basis.
MY VIEW (FAIR VIEW): If you have not taken any spares from company and the amount stands to your credit, no money has been received by you as company is still your debtor for the services rendered by you. Therefore service tax should not be payable at this stage. The moment you take the spares, you have received the money's worth and should pay service tax. On the contrary, if you have taken spares first and coupons are adjusted into value of spares taken, then you have to pay service tax on the adjustment of coupons. However, if the transactions are too regular this way, one fine day the Service Tax Dept. will consider all your spare parts purchased from manufacturer on 'credit basis' as an advance payment for your services and demand payment on receipt of advance basis.
Please consult your ST practitioner and run a test with law on the above basis. Everything is dependent on the way you present your books of account to avoid paying servcie tax at the stage you purchase your spare parts.