Format Number function

[Hiddukel]

I have a function that I got from a snippet that I need a little help with.
It seems to work fine the first time it is called but not the second time.
Here is the code I was using to test it with:

printf_to_ch(ch, "Your current balance is: %s Steel and %s Gold.\n\r",
fnum(1000), fnum(2000));

What this outputs is:

Your current balance is: 1,000 Steel and 1,000 Gold.

When it should be:

Your current balance is: 1,000 Steel and 2,000 Gold.

Here is the fnum function in question:

char *fnum(long number)
{
     int index,index_new,rest;
     char buf[16];
     static char buf_new[16];

     sprintf(buf, "%ld", number);
     rest = strlen(buf)%3;

     for (index = index_new = 0;index < strlen(buf); index++, index_new++)
     {
          if (index != 0 && (index - rest) %3 == 0 )
          {
               buf_new[index_new]=',';
               index_new++;
               buf_new[index_new] = buf[index];
          }
          else
               buf_new[index_new] = buf[index];
     }
     buf_new[index_new] = '\0';
     return buf_new;
}

Any help on this would be very much appreciated.