AMC Related issue.

I have draw a service invoice or Rs. 10000/= against annual mentanance contract on 1st Aug. 2017. Contract period 1/8/17 to 30/7/18. And pay the applicable GST at the time of invoicing. and after three month I need to replace some defective components against the above contract. my question is that time how can I supply this components to the costomer. further require invoice? customer can't pay any against this replacemet. pls suggest me.