Once signed, the contract binds you. Check quickly whether a right of withdrawal is open.
If the purchase contract is already signed, later changes are hard to enforce because both sides are bound. In consumer transactions, however, a statutory right of withdrawal may exist, for instance under section 30a KSchG where a real-estate agent was involved, or under section 3 KSchG for a contract concluded away from business premises. These deadlines are short.
Have it checked immediately whether withdrawal is still possible in time and whether the declaration of registration consent and the encumbrance status are correctly reflected. The sooner, the better.