The golden rule on handling bad agents is usually to hang up and call again. If you got bad service at an office, you should try the phone.

In general, when you buy tickets it comes with certain restrictions according to the fare basis and class booked (in your case B). Full Y fare with no restrictions that is flexible with the airlines is around 3000 USD so when you get a discount it will come with restrictions. You should try again later, once every few days. Look at availability tools to see if a B seat opens up on the flights the guy wants.