regard this issue, there is not much which can be done, no matter what limitations are set, will always be like this. Somoene will be first and will ALWAYS have best exchange rate. If he set low amounts of money beeing able to be converted at once, even this wont help, since, same person can do repeate exchanges but more than this, it will just piss of whole playbase by forcing them to come over and over in game to exchange this. Rate can not become fix, it s too late now for that, so, thats it. It is a football game but also, it has extra features , managing the club at lowest possible expenses comes with an effort.
Who play purely just for this can do much anyway, its limitated by wages, so, such persons will not get rich/richer, max they could do is a few euro. The major impact is made by bigger clubs, and, to me, seems fair, everyone has same chance, exchange date is no longer predictible, who is active, gets rewarded by beeing able to save a few money - in the end, all those big clubs aim for results, managing efficient wages its part of game.