Just to clarify, it sounds like there were two originally unrelated companies, and one is buying the other. If that's so, you should have an attorney get involved, and have him or her try to get in before escrow closes. Depending upon the amount due you from the purchased company, you may be able to force a POS, or "Proceeds Of Sale", agreement, which would stipulate that, in exchange for not delaying close of escrow or taking any overt legal action like a lawsuit, your company would accept payment from funds in escrow before the sale closes.
I used something similar last year, with a company that owed us over $600K, that was being sold to another customer of ours. With the POS and guaranties in place, we recoverd all but $20K of the balance, and *that* was credited off to solve a dispute with a third party. The circumstances were probably very different in the details, but the POS agreement made it possible to recover substantially all of our money.
Regarding the incorrect billing, it depends on *how* it was incorrect. If your company made a good faith effort to provide accurate billing, and information later came to light that made it necessary to correct your billing, then you would pursue the revised amount. If the goods were purchased by the first company for the purpose of sale to the other firm, and the intent was to freeze you out, that means they were acting in bad faith, and possibly fraudulently, though this is extremely hard to prove. This could give your company additional leverage in getting payment from escrow.
