This manga is so full of ass pull plot elements. Not being able to use his gold? There's no way a merchant would refuse the gold, unless he was certain it was connected to a crime in the very same city and thus the merchant would have a very high chance of being accused of associating with a criminal. A merchant's job is to make money, so he would be glad to exchange the dragon gold coins to smaller demoninations if he got a commission fee. It's like the author doesn't know how business works.
Another thing is the 10-15 years limit that suddenly appeared out of nowhere to serve the plot so magnificently. It's hilarious. Imagine a scene:
Slave trader: "Slaves, don't even think about fleeing. If you don't receive magic from your owner for 10-15 years, you will die."
Slaves: "Did you say 10-15 years, sir?"
Slave trader: "That's right! Fleeing is useless!"
Slaves: "Adios, asshole! We'd be lucky to live 15 years as slaves, so better spend that time as free persons!"