I don't think I quite understand how the two interact, but wouldn't your example work with either lexical or dynamic bindings?

    > (let ((foo 1)) ; foo is lexical
    >   (define (bar n) (+ foo n))
    >   (bar 2)      ; => 3
    >   (set! foo 2)
    >   (bar 2))     ; => 4

(Naturally, if instead of set! you do (let ((foo 2)) ...), it gives 3 again. But that's creating a new lexical scope, whereas in your example both definitions of foo are in the same scope, as is the definition of bar. I think.)

OTOH,

    > (define (bar n) (+ foo n))
    > (let ()      ; create a new lexical scope
    >   (define foo 1)
    >   (bar 2))   ; error: unbound variable 'foo'

My understanding of dynamic variables is that bar would see the value of foo just defined, and return 3. Am I confused?

(Tested with guile 1.6.7, which I gather is pretty old. Maybe that's the problem?)

edit: tested this in perl as well, with local to create dynamic variables.

    sub bar { print($foo + shift(), "\n"); }
    sub a { my $foo = 1; bar(2); }
    a(); # prints 2, or errors if use strict is on.
    sub b { local  $foo = 1; bar(2); }
    b(); # prints 3.