modifying code copy data from userform to the next columns

hello 

after searching   long  time  I' v found  this  subject   and  the  file  is for  MR. Variatus  

https://www.teachexcel.com/talk/5189/insert-a-new-row-before-total-each-category-based-on-selected-combobox

he  achieved  this  project  excellantly   actually  I'm new in vba  so  I   hope  from him  or  anybody  help  to  mod  the  code ,   I added two textbox  on userform   PUR and SELL to   add the  values  what  I want  if  I add a new brand  the code  works  insert a new  and  fill the  data but  what  i  miss to be  add values  to  the next empty column every  month add three column   and  if  I  add the  values  to  brand has already existed  it  should not  insert  a new row and  fill it  just  add  the  values  to  the  next empty  columns  because it's existed  I add some  comments inside  the  file  if  it's not  clear  please inform me

thanks in advance