catch?

'Catch' in this use means a problem, maybe a rough spot in a smoothly operating machine.

Catch 22 is a rather well known book, in which various 'catches' were numbered. Catch 22 is one that clearly involves a contradiction. In the story, you could get out of combat duty if you were crazy. The catch was that if you wanted out of combat, that was proof you weren't crazy.

One more question, what does "imaginary number" mean here?
Does it mean the number people can't see?

Imaginary numbers...
It's hard to say for sure without knowing exactly what the author is referencing. I'm going to take a stab at it.
Leading up to the economic collapse in the USA, many of the bankers, investment bankers and traders were using funky mathematic formulas that were so complicated most of them did not understand it. However, using these formulas, they were able to reap unbelievable rates of return, paying themselves obscene fees.
So, I'd imagine...
The author is being tongue in cheek hoping to use the same "magical" formulas to make money appear and debt disappear in his own bank accounts.

Many Thanks!

A imaginary number is a real and legitimate mathematical concept, involving the square root of minus one.

Here I think, the intended meaning is just "fantasy maths".