As said above, you need to ensure her Style is high enough. Also, if a client has rejected a slave once, they will never accept her no matter what changes you made, so hopefully you have a save just before you offered her to him.
Thanks guys for your answers, but it's not a normal contract, it's for the praetor, the style was not the issue (tried it while at max) and you can try many times a dayIf you wait 10 days, you can try again.
Apprently you just need to make the slave a S+ and you can then sell him even though it says A+