Need a little help sifu...

Cat and Ah Siong bought a condo.
Then Ah Siong want to sell off, so cat has to buy Ah Siong's portion. Cat need to refinance the property.

On the RPGT part, which one is correct?
1) Cat and Ah Siong equally pay the RPGT
or
2) Ah Siong pay all the RPGT
Assume in this case is both equally pay the RPGT
When the lawyer in the midst of preparing the S&P, there is RPGT must pay upfront based on estimate, the lawyer firm helps to submit to LHDN.
After 1 year, LHDN sent a letter stated the balance all RPGT to be borned by Cat. Ah Siong no need to pay single cent.
So there is a mistake and went back to the lawyer firm to clarify.... Cat ask for refund portion fees or redo the RPGT.
Lawyer firm doesn't want to admit the mistake. Completely wash hand by saying case has already closed. 

Not a cent discount was given and going the hassle to appeal, what Cat can do?