Find the trick, But pls don't play it on me

Long ago, a clever mathematician used to cheat people. Once he borrowed Rs.4000/- from a rich man.

After a few days, he borrowed Rs.2000/- from the same man.

Many days passed, the mathematician did not return the money to the rich man. The rich man went to the mathematician and asked
to return the money. But to his great surprise, the mathematician replied that there is no need to pay the debt. "See here, friend"
said the mathematician " the sum of 4000 and 2000 is equal to zero, so I do not have any balance to pay".

The rich man took the matter to the court. When the judge came to know
this, he was astonished. He asked the mathematician to
prove that sum of 4000 and 2000 is zero, and not 6000.

The Clever mathematician agreed.

He said " let a = 4000 , b = 2000 and c = 6000"

a + b = c

Multiply both sides by a + b

(a + b) (a + b) = c (a + b)

a*a + ab + ba + b*b = ca + cb

a*a + ab - ca = cb - b*b - ba

a( a + b -c) = -b(b + a - c)

so a = -b

a + b = 0


Hence by putting the values of "a" and "b" as 4000 and 2000 espectively,
their sum is zero, so the
mathematician saw no need to pay any money to the rich man.

The above calculation has no doubt surprised you as it did the udge. So what do u say ...!!! ?
Find the trick, But pls don't play it on me !!!