Modeling conditional update based on existence of a dependent resource

Hi folks,

I am looking to model an interaction in my REST application where the
client should only be allowed to modify a resource (say "B") if another
resource (say "A") exists.

Using HTTP, if resource A exists, I would like PUT /uri/for/resource/B to
succeed. If resource B does not exist, I would like the same PUT request to
fail, responding with a status code that indicates to the client that
resource A does not exist. So what is a good status code to respond with in
the failure case? I considered 403 Forbidden but its semantics say that
"the request should not be repeated". This does not quite make sense in my
application since resource A might come into existence in the future,
making it okay for the client to re-issue the same PUT request for resource
B.

Alternatively (or additionally?), perhaps the client could issue a
conditional PUT on resource B which would only succeed if resource A
existed. How could I express such a condition in HTTP? I considered using
the If-Match request header but that seems to apply to representations of
the same resource (in my example, B) rather than of other resources (in my
example, A).

Thank you,

Shaunak

-- 
"Now the hardness of this world slowly grinds your dreams away / Makin' a
fool's joke out of the promises we make" --- Bruce Springsteen, "Blood
Brothers"