You need to let him have a receipt for the person in charge of their training company. If it is handwritten, a financial stamp is required (please note that the concepts of receipt and invoice are different);

In addition, I suggest that you ask the training class to issue a formal invoice, which is your right and the other party's obligation. If he really refuses to issue it, you can report it to the tax authorities, and he must issue it;

At the same time, please enhance your awareness of protection. When you pay the other party, you naturally need a certificate from the other party to prove that they have received your money. The best proof is the invoice. In this way, you are not happy about the training in the future. Everyone has a certificate, and you can be held accountable, which is also a protection for your rights and interests.