What an awful situation to be in, especially as the pony (small horse) is perfect in every other way. Sadly though, if she is too big, she is too big. Like Michen, I started the process of taking a dealer to court for a similar offence, perfect family horse/safe for all the family, who napped and reared as soon as you put your bum in the saddle, again mine agreed to take the mare back and refund me before it got that far but its horribly stressful and all the time you have a living breathing animal to look after. I hope you manage to get this sorted quickly now without too much more stress