What about borrowing Villa's wording? Thus, you could have, "At the start of your buy phase, you may discard this, to draw a card. If you do, return to your Action phase."
This avoids having to invent something to React to. Even if you have Actions left over, you can just declare that you're done with your Actions, move to your Buy phase, play this, and move on.
That said, I think that this card is pretty weak. Given that it's generally strictly worse than Squire, I think that it could safely cost $2, and even then, most people would only buy it when it's the only +Buy--i.e., for the same reasons they buy Herbalist or even Ruined Market. Maybe you could change the discard bonus to +2 cards?