Mis report in gstr 3b

for Aug 17 of gstr 1m amount of rs 280000 in terms of taxabke value has been shown whereas for same month while filing 3b it was reported as 0. however after 1 yr the same amount has been included in gstr 3b. however in Aug 17 i also did not have enough it available. now my client has received a show cause for disalliwance of its claimed after 1 year and also interest penalty was charged. what to do now