Ok, to reiterate David's comments, the memory never really "deallocates" from usage. This is by design. When you first fire up SQL Server, the memory allocation will slowly grow to what you have established as the "maximum" value in the server properties. It will not deallocate "unused" memory unless you reset the instance. Again, this is by design. There is nothing "wrong" with this.
That said, if you're seeing that SQL Server is using more memory than you have designated, that's something else entirely. If that is what you are truly referring to, have you restarted the SQL Server service since changing the maximum memory allocation in the server properties?