Does the invoice require the buyer’s signature? For that kind of non-payment, how can we prove that the other party did not pay?

1. When the seller issues an invoice to the buyer, the signature of the buyer is not required.

2. For goods sold on credit, the seller can indicate "Price not paid" on the invoice; when receiving the payment, a receipt must be issued to the buyer;

Another way is to ask the buyer to give the seller a debt certificate when issuing an invoice, proving how much the buyer owes the seller.