Sales Return transaction wrongly accounted as Purchases and GST Input Credit taken

Dear All, 

A sales return was wrongly classified as Purchases and instead of reversing the output tax liability against sales return, the input tax of INR 1.8 lakhs was taken under purchases in GST-3B for F.Y 2017-18. Afterward department has sought to reverse this tax credit as the corresponding transaction is not available in 2A. Request you to kindly provide any relevant case law that could help in taking the tax credit, as this is a bonafide mistake by the accountant. 

Pls, share your views.