Thanks for your advise. I should have read it earlier because I was scammed by the trishaw guy yesterday too. This is how he did it.
Before we are in the bike, we agreed with 50,000 dong, he was really kind throughout the journey and I decided to pay him some tips by giving him 100,000 dong. Suddenly, he took out another piece of catalog with price and asked for 1,500,000 dong by saying (15 hundred dong). I was stunned and refused to pay to them and luckily there was a local at the temple trying to help me, after a while they nearly started to fight.
We insisted that we will only willing 200,000 dong otherwise we want to call a police by pointing at a security guard over the road. They then took the money and back off. I would like to place a complaint at the government tourism website but there was no this kind of service.