Not getting paid 2nd half of fee

I was engaged to prepare 3 years 1120 and 3 years bookkeeping for a non-resident of the U.S. who owns a U.S. corporation. Owner is German. I completed the work and took 50% upfront. I have a card on file that I will usually charge but the payment failed. The clients said he would wire the money. He has not. It’s been 45 days since completing the work. My mistake is that I e-filed prior to final payment. Dumb, stupid, lesson learned. I usually have no issue getting paid. Honestly I priced it such that if he stiffs me it’s not the end of the world.

Communication from client has been spotty, he said he needed more time, then he requested a payment plan because of company financial troubles. I said sure, but you need to pay me some amount now and then I’ll take the rest in installments. He’s gone dark again.

I have some idea how I want to proceed, but I don’t have much leverage. The only possible leverage is that he gets the $25k fine for late filing of Form 5472. I told him this was a possibility going into the engagement but that I would request a waiver for reasonable cause. Obviously I won’t do that if he doesn’t pay the remainder.

I’m looking for thoughts from others who have been in a similar situation. What should my next steps be?