Wrong. The Presidential salary is $400,000 per YEAR. Not per month.
And Trump is making far more money with this President gig than that relatively piddling amount. Mar-a-Lago membership fees alone are putting about $8 million a year more in his pocket. And then there’s the money he makes whenever he travels to any of his own properties (such as Mar-a-Logo, Trump Tower, etc.) by the government having to rent rooms at the property for his staff, security, and the rest of his entourage.
And let’s not forget that the Trump brand has been enhanced by his being President, so he’s able to charge more for his other properties as well (not to mention being able to intimidate foreign interests into holding events there if they want to find favor with him). So it was very easy for him to make the bold gesture of refusing the salary.