Let's be hypothetical for a minute. I'm going to use some figures purely for illustrative purposes, they wont be ridiculous but probably won't be accurate either. I'm using £ rather than euros as well, but to be fair there isn't much difference at the minute. Anyway.
The lad doesn't want to come. He wants to go to Sociedad. He is guaranteed to start as No 1, rather than playing cup football for City. We see him as a great keeper long term and don't want to lose out, and then he become brilliant and cost £30m for example, in a few years. We also have first option on him via some third party.
So, we buy him from Deportivo Maldonado, let's say the deal is heavily incentive based like most we do, maybe we only pay £2m up front. We then loan him to where he actually wants to go, with them paying his wages and an obligation clause to buy him. Deal goes through and he signs permanently for Sociedad for £7m. This then negates the incentives in the original deal as he hasn't achieved them and no longer plays for the club, so we're currently in profit having sold a player for more than we paid for him and he hasn't kicked a ball for us. We insert a buy back clause at a fee where we still feel we're getting him cheap (if he goes on to be the player we think), but it's still profit for Sociedad, and enough profit for them to bother being included in this bizarre deal in the first place. If he doesn't develop into what we think he will, we don't activate the clause and we've made £5m on a player that has never played.
If he goes on to be a world class keeper, we buy him back for £14m. Net spend is £9m. Sociedad have made money, Deportivo have made money (they will have future sell on fee entitlements as well), and we've spent £9m on a world class keeper that could now be worth £30m. We either cash in, or he goes in the first team.
This is purely hypothetical, but none of us know the actual terms of this agreement so we can't call the above speculative, and in the other breath say it doesn't make sense. We just don't know either way. However, if it's similar (it won't be the same) to how i've described it above, it's a good move for all parties right?
Time for a coffee.