United Airlines Denied Boarding, Forced Turkish Airlines Purchase

I was set to fly United Airlines (UA194) from San Francisco to Munich, then connect with SunExpress (XQ977) to Izmir. On the day of the flight, I arrived on time at SF Airport but wasn't accepted by UA check-in due to passport issues. UA staff suggested buying a ticket from Turkish Airlines. Amid arguments, a UA Manager directed staff to book Turkish Airlines for the same day, asking if I was willing to pay any price difference. Strangely, I wasn't booked with Turkish Airlines but was instructed to purchase a ticket. I paid $869 (26104 TL) for Turkish Airlines from San Francisco to Izmir, only realizing United Airlines passengers were also on the flight. Both are Star Alliance companies conducting shared flights. Had I been booked with Turkish Airlines, I wouldn't have paid $869, a significant amount for me. I sought reimbursement, though UA rejected it, citing visa issues. I acknowledge my temporary passport issue but believe a solution was overlooked by UA.