Their gain is that they don't have to pay tax on the 300k, because you are paying it.
The correct way would have been that the buyer gives 1.5 million to you, and you pay 300k to the agent for his services after he issues you a proper invoice. Like this you would have 1.5 million income, and 300k expenses, all traceable, so you would pay tax on the 1.2 million profit.
Ask the agent for an invoice for the 300k, but I guess he will just disappear...