I wonder if there's some way to let owners offer/agree to pay only the remaining salary at the time the trade goes through.

As it stands now, we have to guess what that salary will be or have more accounting skills than I've got. But even a good guess or skilled accountant can be very wrong if an offer sits there for a day or two -- or even a few hours, especially if the salary is large. This can violate league rules, even if there's no intent to break them. Most of the time it's clear the intentions are honest, but then again, some owners can try to milk an extra 100K or more to promote players when they're up against the cap.