To me, this looks more like tax evasion since he would be pocketing the money directly. You don’t take five-figure amounts in cash, and it’s completely atypical. The amount is only due upon signing the contract with the notary (that’s when his work is done) and can then be settled with a proper invoice and the subsequent transfer. Whether he gets his money or not cannot be influenced by him at the notary appointment anyway, because the money is only due upon signing, and by then it’s already too late for him.
Conclusion: don’t take any money with you and have a proper invoice issued. You also only have advantages from this, such as easier proof of payment.