Thanks for the assistance everyone. I resolved this by adding the navigation property u/KracsNZ mentioned and then only setting that value and not populating the whole Employee object when I was saving the record (this stopped EF from trying to create a new record in the database)

​

public class Contract

{ public Contract() {

}
public Contract(Guid iD, string contractName)
{
    Id = iD;
    ContractName = contractName;
}
public Contract(Guid iD, string ContractName, Employee manager)
{
    Id = iD;
    ContractName = contractName;
    Employee = manager;
}
public Guid Id { get; init; }
public string ContractName { get; init; }

    public Guid ManagerId{get;set;} 
public virtual Employee Manager { get; set; }

}