Sales return under CST (i.e. Central Sales Tax Act) is allowable as deduction from the turnover within 6 months? If, say, goods are returned in GST by a buyer after 6 months of sales, will it become taxable in CST or GST?

First, find out whether the goods are taxable in GST
or not. Secondly check whether the goods were returned
after 6 months from the appointed day. If the answer to
both happens to be ‘yes’ then the person returning the
goods will have to pay tax in GST.
However, where the goods are returned within 6 months
from the appointed day, no tax will be payable by the
person returning them if the goods are identifiable and tax
was paid under the earlier law at the time of its sale, made
not earlier than 6 months from the appointed day.
The relevant section is section 149.