It sounds like a great deal. But Canada Revenue says it's going after charity tax shelters which offer you a tax credit for more money than you donated. So far 65,000 taxpayers are involved -- on the hook for $2.5 billion in refunds.

"It sounded like a very worthy cause," remembers Patty Mooney about what she thought four years ago when she learned about the Canadian Humanitarian Trust Tax Shelter. It was a plan to help the world's needy get much needed pharmaceuticals.

"So I thought it was like a win-win situation. The way they explained it," she said.

The win-win was that for her $5,000 donation, through a series of steps involving a trust and a second foundation, Patty would be able to claim a total deduction of over $18,000 and receive tax refund of over $8,000. And the world's needy would get much needed drugs.

She got that tax refund just like the trust said she would. She made another contribution in 2006 -- and again got a refund.

Three years later the government looked at the tax shelter and disallowed it -- all of it.

It wanted patty to pay back the refund and nearly $2,000 in interest.

In a letter to participants Canadian Humanitarian Trust says "We absolutely disagree with the positions taken and the conclusions stated by CRA. CRA makes, in our view, a number of serious and glaring misstatements and errors of fact and law concerning the program,"

CHT says it has a $500,000 fund which will be used in defense of any reassessments.

Tax Lawyer Philippe Dioguardi 's advice to anyone who gets a reassessment is pay now -- fight later.

"If you want to fight, fine," he says. "Pay off the CRA first. Get some kind of viable payment plan in place because if you think you are going to win and you don't win by the time you figure it out your bill is triple."

And what about charity tax shelters that claim to be able to give you back more than you donate?

"That's why they are so easy to attack. It's seen as a sham and a fraud and not representative of reality," he said.

Patty Mooney has paid back the government and is awaiting for a second reassessment. And in her opinion.

"I feel now that I'm a victim of fraud if it's not a valid tax deduction to begin with," she concludes.

Before getting involved in any tax shelter get professions advice from a chartered accountant or CGA who specializes in taxes and is not associated with the plan.