You could also use functional dependencies here:

{-# LANGUAGE FunctionalDependencies #-}

class Foo a b | a -> b where

bar :: a -> Int

foobar :: a -> b -> Int

foobar avalue bvalue = bar avalue

instance Foo Int Int where

bar i = 5

main = print $ bar (4::Int)

Here you are saying that the type parameter b is determined by the

parameter a, so GHC knows that the instance Foo Int Int is the only

instance of Foo with a = Int, thus removing the ambiguity.