This is just a FFP dodge, right?
What's the difference between a loan with an obligation to buy and a sale with a delayed payment?
I'm not talking about deals where the players has to meet certain criteria which triggers the sale.
An uneducated guess, ingoring salary:
Scenario A (immediate buy with 5 year contract): You buy the player for 100m and give him a 5 year contract. What happens is you lose 100m of cash and activate him as a intangible asset worth 100m. So at the beginning, you don't lose a penny accounting-wise, you're just changing liquid (cash) to non-liquid (intangible asset). Next you have to depreciate 100/5=20m per year. So in year 0, you have 0 costs. In year 1 20m, in year 2 20m and so forth. This also stretches your balance and may make you more likely to get loan deals from banks etc.
Scenario B (one year loan with obligation to buy, 4 year contract): In year 0, you pay no fee and thus activate no intangible asset. In year 1, you buy and activate him as an intangible asset worth 100m. Then you depreciate him over the run time for 100/4=25m per year.
So your costs look like this:
0 25m 25m 25m 25m
instead of
20m 20m 20m 20m
but you also stretch your balance one year later.