If your special offer was correct than that is what she accepted.
Is it possible the 'missing' $50 is the Airbnb fee? The number you set as the special offer is your total - rates plus special charges. Airbnb deducts 3% of that as their fee from the Host and Airbnb adds 6% to 12% to the guest.
So, rarely are you and the guest talking apples to apples.
The pending payout number should be your net, IOW the accepted special offer less 3%.
Take a look on the confirmation page. You should see the breakout of the total, the fee, and your payout.
Again none of those numbers will be what the guest pays. They will pay the total plus the Guest fee.
See if your $50 is there. If not you may have to alter the booking to add it back on.