Nedbank unfair treatment with international payment

On the 12th of May, I planned to pay my son's tuition at the University of Leeds. I spoke with my personal banker at Nedbank, who advised me to utilize their mobile app. I then did this with her guiding me through the choices and process since I had never done it before. I then paid £2500 through CITIBANK in London. After 3 working days with no evidence of payment from the University of Leeds, I requested through Nedbank if the payment had been made, and they informed me that the 2500 pounds had been paid from their end and provided me with documents to use in my dispute with the University. However, Flywire, which manages the payment process, alerted me on May 24, 2022, that since I chose to pay in pounds rather than rands (as advised by Nedbank staff), their system instantly rejected the pounds I had earlier obtained from Nedbank at the market rate on May 12, 2022. Nedbank then informed me that they will return me the money in rands at the current rate on the 24th, resulting in a loss of R2907. When I asked Flywire why they had declined the payments, they suggested that Nedbank move the pounds to their system. Nedbank now wants to convert the pounds to rands and then sell me pounds based on the lesser amount of rands. They will also charge me for the new process.

I'm not sure why they can't just pay Flywire in pounds.