Rumple said that he didn’t have anything then David said you must have some idea so the sword in the stone idea was from Rumple not from David.
Agree to disagree then. Makes more sense to me that Charming came up with the idea on his own. Rumple looked legit baffled when Snow asked what payment Charming had promised. IMO Rumple gave her that look because there had been no payment, (if there was, we’d have seen it being paid), because he hadn’t given any help. Then he says, “It’s true, your Prince came to ask for aid fighting the Queen. I told him I had nothing to give, and I didn’t.” Even if he’d just given him the idea for the fake Excalibur, he’d have considered that something he’d given, and he’d still have demanded payment for it. He demanded payment just for Snow having wasted his time. Everything about Rumple and Snow’s conversation about the sword felt like Rumple saying, “Honey, your Prince fooled you, and you were too silly to see, but even I can see it, even though I had nothing to do with it.”