Problems
The amount received is not the one announced
First check the quantity actually deposited: it is often lower than the one typed at the sender.
The most frequent cause is not a calculation gap but a deposit different from the order. Many platforms deduct their withdrawal fee from the amount typed, and wallets on UTXO chains adjust the amount downward when you send the whole balance.
In that case the net is recomputed on the quantity actually received, at the locked rate. The spread scales down proportionally, but fixed fees do not move: on a large gap the relative share of fixed fees rises.
If the quantity received does match the order and the net still differs, compare the receipt with the original quote and open a ticket attaching both. The lines must sum to exactly the gap between gross and net.
Going further
This article does not cover your case?
Open a ticket with the order number and, if it concerns a deposit, the transaction hash. Those two items are usually enough to settle it with no further exchange.
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