Problem With attributes and quantity

[rohitkpatil]

I am having problems with attribute and product quantity in stock.

i want to offer buyers different prices for payment through credit card and bank transfer. I want to charge them certain price say $10 for one product with Credit card, and for the same product want to charge $8 if they choose bank transfer.

What i did is, i made attribute group with payment mode. And created two options, Credit card and other one bank transfer. If they select credit card (which is a default option) nothing happens as price is kept same $10, but if they select bank transfer, price reduction takes place and price becomes $8.

Now when i keep these attributes, if i have a product in quantity 2 in stock, it becomes 4 quantity automatically considering 2 for each payment mode. if i keep 1 quantity for each payment mode that will not serve purpose. As if one gets sold with one payment mode, the other remaining quantity is available through other remaining payment mode only. So selection process not happening keeping exact quantity in stock. How to solve this?