This is likely to be an Apache problem. You can probably fix it with a RewriteRule that simply adds a slash:

RewriteEngine On
RewriteRule ^/foo$ /foo/ [R]
[download]

If there might be arguments after foo, adding something like this might work:

RewriteRule ^/foo[^/](.*)$ /foo/$1 [R]
[download]

These aren't tested, but you get the idea.