Hmmm if its a corrupt payment and you pay it you perpetuate the corrupt practise. Immigration appear to be cracking down on this stuff might be an idea to ask his name and employee number then offer to pay the hone visit fee at the office whilst getting a receipt.
You need to have ฿20k or the equivalent in another major currency because the immigration officials are within their rights to ask to see it. Proof of funds to support yourself is a prerequisite to being granted a stay in the kingdom. If they ask and you don’t have it you will be refused entry.