Zencart: Checkout Currency VALUE

I need someone to help me change the default value of ONE currency during checkout. Example: MYR is default at 1.00, SGD based on exchange rate is 3.05MYR What I need is to change SGD value to 2MYR (instead of currency exchange rate) during checkout when using PayPal or any other payment option available - but still checkout in SGD. **So a product priced 100MYR will be listed as 50SGD and checkout at 50USD. I have tried changing it in currency exchange rate - item is listed as the desired value, but after checkout, it still changes back to original currency rate (based on current exchange rate). Order tested using PayPal. I have a test site which you can run on before implementing it to live server. Cart: Zencart 1.5.5e