If you put stuff in the anvil to rename it or to merge it, you only need to go with the mouse over the "output" place and press "Q" (the default drop key).
The item will be processed, even when you dont have enough XP or even none.
It will take the XP you have, but you can use it for free actually.
I marked this issue as private, as it is pretty much a gamebreaker and I dont think you want other people to hear it ^^. If this level of "exploiting" is not high enough, you can just mark this issue as public and dont need to write a message down below this, I know I marked it wrong then.