What is the value of a 1934 A US 10 dollar bill with a yellow seal?

The yellow seal indicates that you have a special bill printed for use by Allied troops in North Africa during WWII.

1934 A North Africa bills are the more common variety; as of 11/2012 its retail value is $20 to $60 in average condition and $200 or more if uncirculated.

If there's no letter next to the date you should have it examined by a currency expert to make sure it's genuine. These early bills are quite scarce and sell for $1200 to over $10,000.

Why yellow?

Yellow and brown seals were used on bills distributed in war zones and threatened areas. The different colors could be easily identified and the bills would be declared worthless if they fell into enemy hands.