Lost resources
When you cancel an “in progress” crafting, or training, the resources are deliberately lost - per Devs ( see Notes), so Iron or food would be lost.
Repurposing your builder
The only case I can see for urgently needing to undo a builder would be incredibly niche.
Most cases it it just better to move on with your game.
But a reset button could be added to the game
( see Notes ).
Notes
(MASTER THREAD (Solved) - Training Camp Lost food/recruits at withdrawal )
(Ability to reverse/undo an accidental feeding of a hero - #24 by Gryphonknight)