I can't be "Bittrex specific", but i can provide a general answer to how it would work and have seen it work across a half dozen systems that are crypto specific and dozens of financial systems that are non crypto specific:
When you set up a sell order (or just click sell) it's usually not a "1 and done" situation. You are selling 'x' amount for at least 'y' value. As the system matches up the purchases of your 'x' amount to buy orders, a fee is applied when the match is made and the "trade" occurs. The fee can be taken out of BOTH sides of the trade. You have a .25%.. the buyer may have a .25%. That means if the trade completes, whatever you are "due" they will remove it before handing it over. THEY get BCH. The person buying it get's a fee too.. they paid in BTC. Once the trade completes, they pay their fee in BTC.
Sorry this isn't specifically a Bittrex answer, it's a "how these things normally work". I can't imagine how else it could be accomplished as it would mess up all normal accounting practices.
In sum, to maintain accounting: The trade is completed. Fee's are assessed against the results.