public async Task UpdateAsync(Guid settlementId, Guid userId, Guid expenseId, string name, decimal cost)
{
var userExpenses = await _settlementRepository.GetExpensesOrFailAsync(settlementId, userId);
var expense = userExpenses.SingleOrDefault(x => x.Id == expenseId);
if (userExpenses == null)
{
throw new Exception($"Expense with id: '{expenseId}' does not exist.");
}
expense.SetName(name);
expense.SetCost(cost);
manager.Settle(await _settlementRepository.GetAsync(settlementId));
await _settlementRepository.UpdateExpenseAsync(settlementId, expense);
}