You will get your $6,000.

The $2,400 will have nothing to do with it. 

The tax return will have three new lines:

A)  Figure your credit based on 2020 income and qualifying children.

B)  How much "advance" credit have you already received?

C)  Subtract B from A and enter the result,  but not less than zero.  If it's more than zero, it gets added to the refund you already have.  

Some people will get more "rebate" credit on their 2020 return because their income was over the 75K/150K limit in 2019, but not in 2020.  Or because they have more kids in 2020 than in 2019.  But no one will have to pay back the credit if the 2020 amount is less than the 2019 "advance."