Buyer made aware of dishonesty in tawlia

The buyer may reduce the excess from the price.
Yes. According to Imaam Yusuf (rahmatullahi alaih) the buyer may reduce the price, relying upon what the seller claimed - this applies to both Murabaha and Tawlia. There is no choice of invalidating the contract. According to Imaam Muhammad (rahmatullahi alaih), the price cannot be reduced, in either case, however he has a choice in both cases (either to accept the agreed price or to return the goods) (Kitaabul Buyoo).