The Brazilian striker joined the La Liga champions earlier this summer having been linked with the majority of Europe's elite clubs.
While Barca always seemed the likely destination for the 21-year-old, Rosell revealed that Neymar had even undergone a medical with their bitter rivals.
"When we reached a pre-agreement with Neymar's father, the player had already passed a medical with Real Madrid," Rosell told TV3.
"We joked that we could save ourselves the money of doing the tests ourselves. When we were already signing the contract, Madrid was sending emails to Santos."
Neymar has scored 24 goals in 40 appearances for Brazil.