Laravel: ignore case when using string primary key in relationship

Question

I'm working on a new project using Laravel where I need to use data from a legacy database(so please ignore the database design).

This database is using the username as primary key:

The User model has a One To Many relationshio with "ChangeContract" model

/**
     * @return \Illuminate\Database\Eloquent\Relations\HasMany
     */
    public function changesContract(): \Illuminate\Database\Eloquent\Relations\HasMany
    {
        return $this->hasMany(ChangeContract::class, 'Username', 'UserName')->latestFirst();
    }

The current user has multiple contract changes:

But only two Contract Changes are returned by the relationship, the ones that matach the primary key(user name) exactly(case sensitive):

   "changesContract": [
        {
            "type": "Contract Change",
            "username": "FroneaBA",
            "date": "2017-10-01",
            "endDate": "2018-06-30"
        },
        {
            "type": "Contract Change",
            "username": "FroneaBA",
            "date": "2017-07-01",
            "endDate": "2017-09-30"
        }
    ],

I need also to return the items where the key is froneaba. Is there a way to set the primary key to be case insensitive?

P.S: From the database client, if I write the query by hand, all the items are returned:

SELECT * FROM changescontract WHERE username='froneaba';