I've never heard of such a thing, and don't think they could ever keep such a thing secret without the less affluent members getting wind and getting upset, plus I think the rich and faithful members would find it troubling if they were offered a discount while poor people, who have less resources in the first place, wouldn't be. Such would have been very troubling to me if I had been an honest, faithful, wealthy member.
I believe it to be BS. But there are ways to cut your tithing and still be a full tith payer. It is by way of investments, reinvestments, Retirement funds, and of coarse owning a busness whose income is tithing free. One pays tithing on the income they draw form their company and the car, home, travel, etc is paid for by the company. Thus the rich pay less, and save more.