I don't know about that. I have seen many teams report this and felt it my self when I sold my foreign high salary players that merch went up a lot even though you had cut your teams salary in half.
This is a fairly ridiculous situation in itself. As you say, merchandising goes up when you sell your best foreign players. In essence, if you intentionally make your team worse, your merchandising goes up. I hate to use this word in general, but that is just stupid.
The merchandising you get for home players in that situation should always be there, it shouldn't increase because you sold a better foreign player.