When buying a second-hand house, the value-added tax invoice amount is different from the contract amount, which may be caused by the following circumstances:

1. Tax policy restrictions: According to the regulations of the local tax authorities, the value-added tax invoice may only be issued according to the actual value-added part of the house, but not according to the total contract amount.

2. The contract price is different from the actual transaction price: in some cases, the buyer and the seller may agree on a lower price in the contract to reduce the tax burden during the transaction. In this case, the amount of VAT invoice will be based on the actual transaction price, not the total contract amount.

Please note that the above are only some of the reasons that may lead to the difference between the VAT invoice amount and the contract amount, and the specific situation may vary according to the differences of regions and individual transactions. Therefore, it is suggested that when you encounter this problem, you can consult a professional tax consultant or directly seek help from the local tax authorities.