1040-X basic, (hopefully) ,question

I'm confused.

1040 sent, let's say 500 USD refund expected. Late 1099 comes in need to send 1040-X with refund now recalculated at let's say 450 USD.

So, am I expecting:

a - A refund for $500 and then I send them a check for $50?

b. A refund for $450 straight out the gate and that's it?

I'm thinking the way the 1040-X form calculates the numbers the IRS are expecting a payment from me that covers the 'refund' excess?

I also see references to 'waiting for the IRS to accept your 1040 before sending in the 1040-X'. How is that verified?.....USPS return of the certified mail receipt postcard.....or...wheres my refund....or what?

 

Thanks