You can Sell ammo that's in Inventory. Just find the Crate you want to sell ammo from, go to the Inventory tab and click the 3-dot menu next to the caliber you want to get credit for:
Please Note: If you're on mobile you may need to swipe your screen to the left TWO times, once to see the 3-dot menu and again to see the options. Or you can turn your phone landscape to see more.
Just click the yellow button marked "Sell Ammo" at the bottom of the confirmation page to complete the transaction.