Real sale
The minhag is to pay the rav for the efforts involved in selling chometz to a non-Jew. This lends extra legitimacy to the sale, showing that it is a true sale, not a mere ruse or ceremony, as he is paying for the transaction, in the same way one pays a lawyer’s fee when drafting a contract. This is also an opportunity to show appreciation to the rav for what he does all year...
Status of a po’eil
Some write that there is another advantage to paying the rav. If the rav is just a shliach, we can only assume he carried out his shlichus due to a chazaka, but for deoraisa issues, we do not rely on chazaka. Paying the rav makes him into a po’eil [hired worker], in which case we assume he did his job even without the mechanism of chazaka (תשובות והנהגות ח''ב סי' רי''ח ).
