Save the platform terms that applied when the contract was signed, then add each later version. Keep emails, partner-account messages, notes of calls and any stated dates together. A short timeline can show when the hotel offered a lower direct rate and when the platform responded.
Capture the hotel website and platform display for the same stay, number of guests, room and rate. Keep the booking steps, payment method, cancellation terms, included services and any fees. If the platform referred to ranking or visibility, also record the search, time and displayed position.
File statements and commission records separately. That makes it easier to tell whether the issue concerns a parity term, a price difference or an accounting question. A clear contract version, the specific demand and a comparable set of offers usually provide a useful starting point for the next review.