Selling Jersey Numbers

When Clinton Portis joined the Washington Redskins, he wanted to wear #26. But Ifeanyi Ohalete already had the number. Easy enough: the bit player Ohalete sold his rights to the number to superstar Portis for $40,000. Things got interesting when Portis paid only half and Ohalete was cut from the team. Is Portis still required to pay? This almost ended up in court, but the players settled and Portis will pay $18,000 of the $20,000 he still owed. Apparently, trading numbers not uncommon, but why do teams give players rights in their numbers? Why not just assign #26 to Portis and give Ohalete a less popular number?