Need to return a sum of values based on criteria from another file.

I have two files, each file has a unique text ID in column A, in column B of
one file is a sales figure in the other file, column B contains a debt
figure. The files are generated by a system each month automatically and I
can't get the producer to provide a file with sales and debts in one file, I
also don't want to insert a column in one of the above files and add a look
up to the other.

So far, I have a simple formula in a separate file
=SUMIF([Book3]Sheet1!$B:$B,"<=100",[Book3]Sheet1!$B:$B) that returns the sum
of records that are less or equal 100 from one file, this could for example
be adding up records that have unique IDs of say A123, A345 and A678. What
I want to do is now return the sum of debt for the same 3 accounts albeit
these are in another file.

Hope this makes sense, I've been trying to solve this most of the day!

Thanks, Rob