The Excel function you want is ABS()
To compare two numbers in cells A1 and A2 irrespective of sign, use this formula:
If you want help on the rest of the issue, some more information would help.
Are there ever more than one match, say:
If so how are these handled.
Do you want your list of unmatched values in a different location, or do you want them in the same place.
Is this a one time or occasional process where it is OK to go through a few steps to get the results,
Is this a frequent process that would benefit from automation with a macro (with it's associated overhead of entering code, and managing it if circumstances change)