hi new to this forum am in first stages of trying to get money back from other halfs bank. can i do the same with egg, if so whats the first step thanks

this is my first post and i wonder if anyone can help me my partner has a car from welcome and we were told last week that we could have a settlement figure of half the cost of the loan. this would mean giving the car back. more than happy to do this other half rang them today to chase them, as someone was supposed to be coming to view the car. basically we were told the settlement figure was wrong and we now owe another £3000. how can they do this?