stage wise payment for Service orders

Hi techies,

stage wise payment for Service orders. Following items are required in SAP while creating & releasing Service Entry. Matter has detailed by following example:

Stage wise payments for an item for eg: Rate is Rs 100 per Rmt.

Item shall be certified as 100% and released stage wise.

Say 60% release on deployment of machinery

Another 20% on execution of work

Another 10% on submission of final bill

Balance 10% as Retention Amount.

Please provide me the solution for release of part rate as explained above.

Thanks in advance,

NB