Strange decision. OK for the one time per season for each player. That will avoid the process of making mutant players.
But, allow negociations only the last week of season is a nonsense, because of the -20%. It's too expensive in tokens to buy a 99% player at this moment of the season.
I usually never buy a player in this week, with negociations, transfert or recommanded players. I do it always in the two first weeks, rarelly in third week.
Or you must allow to buy players better than 99%, else it's totally a waste of tokens.

Most of transactions in negociations are made in the first week. And concern usually "old" players. Some of my regular opponents build their teams in the first week of the season with negociations.

In addition to that, i must admit that one of my pleasure is to make scouting in the upper division than me, to search young good players that are coming out of the academy (my academy usually always giving me craps )