index.cs

// maps

from u in docs.Users 
select new {
    u.Id,
    u.Name,
    Amount = 0
}

from c in docs.Charges
select new  {
    Id = c.User,
    Amount = -c.Amount,
    Name = default(string)
}

from c in docs.Credits
select new  {
    Id = c.User,
    c.Amount,
    Name = default(string)
}

// reduce

from r in results
group r by r.Id into g
let user = g.FirstOrDefault(x=>x.Name != null)
select new 
{
    user.Name,
    user.Id,
    Amount = g.Sum(x=>x.Amount)
}