"Ah, boss, that thing you're asking me to do is, technically speaking, theft. In addition to exposing us both to liability, if it is ever discovered, for example when a potential acquirer or large corporate customer does a line-by-line source code audit like they frequently do, our company is finished."
Boss plays ball: "Great, glad we're on the same page."
Boss directs you to steal it anyway Option A: "OK boss, if you're sure it's OK, I'd like you to send me a written email or memo instructing me to copy this code from their computers to our computers. You should CC it to $OWNER so that he knows we've taken action on the IP agreement we made with him, or if you'd prefer I can forward him a copy."
Boss directs you to steal it anyway Option B: "Alright boss, push or shove time. You can, probably, convince someone here to copy that code for you. If you do, I will hand in my resignation immediately. Think it over."
Boss directs you to steal it anyway Option C: "I quit, effective immediately."
n.b. In many US states you can get fired for just about anything, including "no reason at all." Regardless, the market for engineers is super hot right now. Work for someone who isn't an abusive jerk, because you're highly likely to find that he is not solely abusive to business partners.
n.b. #2: It is almost impossible in the US to get away with withholding salary for work already performed. One complaint to your state's labor relations board and he'll wish he had never been born. This assumes you have an employment relationship with the new evil boss. If you're working without a contract (don't ever work without a contract), you're probably best served by just writing off the three days or whatever you have invested in this job.