Zend certified PHP/Magento developer

How to checkout with Display currency?

My base currency is USD, but if display currency is EUR then user should be able to pay in EUR rather than USD.

I know such extension is available but is a ‘paid’ extension. I am trying to overcome this problem by writing the code myself but I am stuck as where to begin. I tried to change the Quote and replaced Base & store currency with the Quote Currency i.e. (display currency).

However, upon saving the quote and again getting it from session it shows base currency rather than display.

So how can I achieve this? Can I disable base currency? or perhaps during calculations swap base currency with the display currency?

P.S: I have already searched but couldn’t find authentic solution, therefore posting a new question.