I booked a ticked from an travelling agency recently and its for August month. Now I want to do changes with travelling (returning) date and destination city due to some personal problem. I contacted travelling agency but they are asking me to pay $775 for making those changes. Is this a scam because its too expensive. I tried to do modification through Qantas airlines page but it says that I need to contact agency. What I should do now. Please give me some suggestions. I don't want to pay that much. Will airlines can help me even though I haven't buy tickets from them?

I booked a ticked from a travel agency recently, for August. Now I want to change my return date and destination city, due to some personal problem. I contacted the travel agency but they are asking me to pay $775 for making those changes. Is this a scam? It's too expensive. I tried to do the modification through the Qantas airlines page, but it says that I need to contact the agency. 

I don't want to pay that much for the change. Will the airline help me even though I haven't bought the tickets from them?