Custom Prices based by selection

[Niko]

Hi All.

I have a client that is in need of a shopping cart that can handle the sale of gas and gas cylinders.
The trick is that the user should be able to select that they are returning an empty cylinder deposit, and the price adjusts lower. I have setup a cart that increases the price, however, this is proving a bit more tricky for me.

The formula is as such:
Product A = $100
If a user purchases product A, but has an empty cylinder to return, it will cost $50
If a user purchases product A, and does not have an empty deposit (cylinder), it will cost $100

It is an "extra", but one that if the user selects, it will deduct the total price.

Additionally, a user may have more than one return deposit, therefore, decreasing the total price of Product A.

I hope this makes sense, and it is not too difficult to achieve. Thank you for your response

[Niko]

EDiT -

Some helpful understanding.

I would like to achieve the ability to have a variant that has its own quantity selector, that affects the overall price.
So for example. If we were selling Tshirts, and you can add more than one badge, so you would select 4 badges and if affected the price. - In my situation, I would need this to be a price negative affect.
Users should be able to select how many deposits they would like to return, affecting the price by applying the credit to the cart the more deposits are returned.

I hope this gives more understanding.

[Milbo]

So you want the substract and not to add? you ever tried to use the minus sign in the customfield price field?