On her move, Alice trades with Bob: Alice gives 2 wheat, Bob gives 1 sheep.
Then Alice does her calculations again and decides it was a wrong move. She wants to trade her sheep back for one wheat, and Bob is willing to give her back 1 wheat for 1 sheep. Alice does nothing else between those trades.
Other players object, saying it's a catch to work around "no giving away cards for free" rule, and Alice and Bob might be conspiring.
I (being Bob) know for a fact it's not true, and the game rules don't explicitly forbid it, as far as I can tell.
Is it a valid move?