I would think that all the details of the order and purchase to include your responsibilities as it relates to the transaction would be worked out before the order to build was placed and not negotiated along-the-way. Is the 5000 up front determined within a written policy?
But things as they are based upon your expalaination, and as others have mentioned, I would not front anymore money until the rig is on the lot and ready for inspection.
Does this dealer have previous complaints or reviews that would reflect negatively on their ability to deliver or handle their business with integrity?
Let us know how this turns out....I’m sure your good karma with being honest about the accounting error will help out if needed.
:)