You fail to see the real problem here. The backlog might well be the reason for the losses!

The contracts for these planes has already been signed. This also means the amount of money they will get, or more specifically the amount of DOLLARS they get, it set in stone. Now that the dollar has fallen in value, the income has also fallen in value.
…

True, their income has fallen in value in terms of currencies that have risen against the dollar. Some of their costs (those in dollars, e.g. for US-built engines) have also fallen.

About that backlog: Many things to consider. e.g. what’s the lead time on components? Different for different components. Could be an opportunity to switch to lower-cost sources by the time parts have to be bought for some of that backlog. Contracts are now being signed for deliveries in about 2015. What level will the dollar be then? Short-term, it may well fall further. But in 7 or 8 years time?

BTW, not all of Boeings costs are in dollars, especially for that massive backlog of 787s, for which manufacturing has been deliberately spread around the world, with large parts made in Italy, Japan, etc, so Boeing isn’t laughing all the way to the bank on everything. It has a cost advantage on current deliveries, but Boeing knows very well that can change, & quite quickly.