In theory COMBIN works with integers in Excel (2003 and 2007).

So "=COMBIN(9,3)" gives 84 and so does "=COMBIN(9.7,3.6)".
Similarly "=COMBIN(9,3)-84" gives 0.

But "=COMBIN(9,3)-84-0" gives -1.42109E-14
and there are many similar examples suggesting that some sort of
rounding is involved.
The sign can even change so for example "=COMBIN(15,3)-455-0" gives
5.68434E-14

=IF(COMBIN(9,3)=84,"same","different") and
=IF(COMBIN(9,3)-84=0,"same","different") do not give identical
results.
Nor do =COMBIN(9,3)-67-17 and =COMBIN(9,3)-68-16.




It gets stranger: