Rules on relationships between different SQL tables

You can enforce this relationship automatically at the database level using a composite key. However, you won't be able to use guids. If using guids is a requirement on your problem, then this solution won't be useful to you.

The solution shown below will work on any relational database, without extra code:

create table party (
  id int primary key not null
);

create table contract (
  party_id int not null,
  contract_no int not null,
  primary key (party_id, contract_no),
  constraint fk1 foreign key (party_id) references party (id)
);

create table damage (
  id primary key not null,
  party_id int not null, -- not nullable
  contract_no int,       -- nullable
  constraint fk2 foreign key (party_id) references party (id),
  constraint fk3 foreign key (party_id, contract_no) 
         references contract (party_id, contract_no)
);

Note: The SQL standard defines three matching options for partially null FKs (none, partial, full) but each database is a liberty to use any of those.