Owners of professional sports teams could gain tens of millions of dollars in the values of their franchises because of a clause in an export-tax bill in Congress now.

The bill is set for a joint vote of both houses of Congress which have already voted to pass it.

If law, owners would be allowed to write off the full value of their franchises over 15 years.  

Existing law generally limits team write-offs to only the value of player contracts over three to five years.  But the new law would expand write-offs to television and radio contracts.

"They're doing very well in this," said Robert Willens, a managing director at Lehman Brothers.

Willens predicted the bill would become law in October.