It's not the general theory of relativity ffs.
You have 50 mil € to spend on players. You arrange transfers of 5 players for 10 mil € each, paid in 3 installments. So that would be 16,6 mil € per season. However, you count that you spent the whole budget and pay the installments from that budget in the future.
Next season you calculate a new budget from which you will sing a few players in installments, and you do the same. Otherwise you would have to calculate the previous years transfers in this years transfer budget if you counted installments only.
If it were did that way (as you suggest): let's say we have 50 mil € next year, we pay the first installments for Matri, Quag, Aquilani, pepe, for, lets say, 15 mil €. That would mean that there are 35 mil e left, right? Wrong. Because if you looked at it taht way you would also need to deduct the installments for Diego, Melo, rkasić, Bonucci, etc from the transfer budget and again you'd end at 0.