purcahses rerurns in gstr 3b

hi, the purchase returns are excess to puchases done in june month. due to this the total tax is showing in negative in tally...how shud i file the return in this case.. i dont have any previous input also...can i show the tax amount relating to purchase returns in outwards column. if so, what value is to be shown in taxable value, cgst,sgst and igst.