According to U.S. law, the recipient (if willing and able) may legally reemburse the donor for lost wages during surgery and recovery, and for travel, lodging and other expenses directly related to the donation. You may not legally make any financial profit. Perhaps there was a misunderstanding with the hospital? It is true that the recipient may not "pay" you for the kidney, but the recipient CAn reemburse for losses, and is free to organize fund-raising events, etc. for that purpose.
best wishes,
Fr. Pat