I would be flying from PRG-SIN in a couple of months in Business Class considering how much one way tickets cost in Y, but I was thinking, is it possible to book a return ticket and refunding the inbound flight.
Looking very quickly at the fare tariff Air France has filed on PRG-SIN, the cheapest return fare in business class is 1777.00 EUR and the cheapest one-way fare is 1915.00 EUR. These figures exclude taxes, fees and surcharges. The cheapest fare has a change fee of 240 EUR. Therefore the cost of cancelling the return flight would probably be 378 EUR, however I have not validated this information so please check before you purchase.
A better strategy is to consider your true itinerary here. Are you planning to fly to Prague to Singapore and never ever return to Europe? If you do plan to return to somewhere in Europe at some point, you might be better purchasing the actual itinerary you want to perform.
If you are determined to indulge in throw-away ticketing, your best strategy is to buy a return trip outbound in business class and return in cheapest economy. The fares may be combined to form round-trips on a half-round-trip basis, so the price would be half a business class return fare + half an economy class return fare. You may have to telephone AF to buy this as you cannot do it online. I can price that (inclusive of fees and tax) for about 1433 EUR.
If you book a flexible fare, you could potentially modify or even cancel it. But I doubt that it will be 100% free. It usually depends on the fare purchased.
Another option would be to purchase an Y ticket and then apply for an upgrade. It might work and you would lose less if you can't be refunded on the return flight.