jemandem ein X für ein U vormachen
to deceive someone; to pull the wool over someone's eyes
Origin
Goes back to Roman numerals: the V (= 5), from which our modern U also derives, could be 'falsified' into an X (= 10) by extending the strokes, thus doubling the amount - e.g. by moneylenders or innkeepers on the tally board.
Building blocks
Words in this expression
Examples
Lass dir von dem kein X für ein U vormachen!
Don't let that salesman pull the wool over your eyes!