sfvoyage, SQ won't 'know' about your incoming flight on GF if it has been completely separately ticketed. In this case you'll very likely have the THB 700 included. Certainly if you book online, and very likely even if you do it manually with a TA - unless you buy both tix with the TA and have a single 'booking' - but this might all be more expensive than the USD 20 you're trying to save!

Not sure if it'll be possible to get the money 'back' from SQ, since how do they really know that your a transit pax. I guess you can only try at the airport - but that seems a bit of a hassle for USD 20.

I guess you should be calling SQ now, to make arrangements to seek a erfund either in BKK (unlikely IMHO) or after your travel is complete.