Balthasar was a Catholic priest in his Office. But all Catholic clergy (priests and Popes alike - as well as ordinary laymen) are permitted to exercise the role of private theologian, and explore (and even advocate) positions not held by (but not contrary to) Catholic doctrine.
To whatever extent Benedict-16 (Ratzinger) agreed with any non-doctrinal positions of Balthasar, he did so in his capacity as a private theologian (the same capacity that Balthasar himself exercised).
Prior to becoming Pope, however, Ratzinger hardly “kept to himself” very much. He is (by FAR) the most published author to become Pope. We know quite a lot about what Ratzinger thought - he made no secret of it. He wrote MANY books about it.
Balthasar died in 1998, years before Ratzinger became Pope in 2005. Ratzinger had PLENTY of opportunity to make his views known (as he was apt to do), even before he became Pope (at which point he would probably scale back his “private theologian” role, to avoid misunderstanding).
Ratzinger had (at least) a seven-year window of opportunity to accept any of Balthasar’s ideas without serious consequences. If he did so in those years, he would have made it known (as was his practice). I am not aware of any such agreement in this regard.