The fountain meeting just doesn't add up after we've been shown and told that they made contact at the mosque and when the diplomat slipped Walker a message in the money.
They did mention that it could have been hidden in his shoe that they missed or I'd believe that Brody did slip it to him knowing that he would kill himself and give Brody some peace.
That was before they knew the CIA were after Thomas Walker, so he was able to hang out in the street like a homeless dude and he was able to show his face at the mosque. Maybe they had a new plan after Walker became wanted? I don't know, could go anywhere, haha.