You can both buy and sell Singapore Dollars online from a trusted currency exchange marketplace such as BookMyForex. You just need to visit bookmyforex.com and select your city, currency (SGD in this case) and the amount that you want to sell online. You need to upload your Passport copy which you can do online. Once your SGD to INR conversion order placed, a representative of BookMyForex would visit your place and give you equivalent INR and take SGD from you.