OK, lets try this.

Select Table1
Scan All
  llFound = .F.
  lnLen = Len(ID_Field)

  Do While !llFound and lnLen > 0
    llFound = Seek(Substr(ID_Field,1,lnLen), "Table2", "ID_Index")
    If !llFound Then
        lnLen = lnLen - 1
    Endif
  EndDo

  If llFound Then
    * Found a match - do some stuff here
  Else
    * no match - do some other stuff
  Endif
EndScan

This will scan table1 and look for a match in table2 for the full 6 digit ID. If it is not found it will remove the leftmost character until a match is found or until there are no more characters to remove.

It is up to you to decide how many characters are allowed to be removed by adjusting the length of lnLen at the top of the while loop.

HTH

Liam

PS This code is completely untested but you get the idea?