Invoice format for jewellery

my second query is billing format for jewellery dealer. Suppose a customer brings old gold in any form whether it may be bullion or in form of jewellery for convertion of new jewellery for e.g. a customer brings 100 gms of old gold and his new jewellery is of 150 gms then how should we raise the invoice i.e. for entire 150 gms and show 100 gms as urd purchases or 150 gms less 100 gms and raise invoice for on 50 gms which format is correct and if the second one is correct then please give reference such as notifications, act,rules, circular