Barca are not going to pay a club €1m to take a €12m rated player off their hands for 2 years to give him experience. While Bojan is no world beater, he's a decent player and there'd be more than enough interest in him, if he was available on loan, for Barca to probably get paid for loaning him out.
Also, how often do you see clubs loan out 20 year olds for 2 years? If Bojan was so highly regarded at Barca that they're willing to pay to give him experience then he wouldn't be getting loaned out in the first place.
The truth of the matter is that this deal is either a crafty way of boosting Barca's figures in regards to FFP or it's been worded wrongly and Barca aren't obliged to offer Roma €13m in 2 years, they just have the option of doing so if they wish.