How to compare two columns with data-type of decimal(15, 2) using "<" operator on sql-server with laravel

I tried comparing two columns with data-type decimal(15,2) using laravel and got the error [SQL Server]Error converting data type nvarchar to numeric. how do i fix this?? my code below

self::where('user_id', $user_id)
      ->where('amount_due','<', 'amount_repaid')
      ->where('loan_type', $loan_type)
      ->get()

via Chebli Mohamed