In our first game last night we had a player that had a helper in the Palas and the Forge.
At the end of the game he had 4 stones and 2 silver.
He converted one silver to a stone to get the maximum of 5 stones for scoring his helper in the Palas.
Can he convert his last piece of silver (even though he has already maxed out his Palas score) to increase his Forge helper's score?
We ended up saying no on this basis:
1. the conversion of silver is for scoring the Palas helper
2. maximum score for this helper had already been reached
I would say of course he can make two trades in the Forge. There is no rule that says you must use your current supply of resources before you can use the forge. It simply says you can use up to 5 to score points, and if you want to convert silver you can.
For his 5 in the Palace he's allowed, he can use his 2 silver, and 3 of his current stone.
It's just 1 extra point for him, so it's not like this is an overpowering move.