1. You may get aggreement to hold back some money until the snag has been completed, or
2. get the vendors solicitors undertaking that the snag will be completed after closing
1. Most unlikely.
2. No way is a solicitor giving an undertaking like that. The builder might but the solicitor will not.
Its normal for a builder to insist on closing even if there are minor items still outstanding and to confirm , normally in the original Contract, that those items will be dealt with.
mf