How to treat transaction regarding jwellary purchase from customer

Tushar Gupta (26 Points)

27 June 2020  
dear respect sir
I am engaged to jwellary shop business
my question is suppose i sale a jwellary to customers and take gst on this sale value.
after 6 months that customer come to my shop and sale same jwellary to me then
what is rule on gst
how to record this transaction
what type of invoice raised