There is simple solution - salary should be payed directly after transfer.
Unfortunatelly not... if a mutant costs 1 million and has 500k salary, the total cost to buy him for one week is 1,5million...
if you make the club to pay one salary at the moment of purchase the result will be that mutant prices will drop by that amount... so it will be 500k transfer fee + 500k salary + 500k salary for next week (the sum will not change)... it will only make things worse as the transfer fee will be lower so more team will be able to buy that player, even in lower leagues...
what I wrote already happened many years ago in another online manager game - Hattrick, believe me it will be the same way here if the same "solution" is implemented