Sum for one to many, change NULL to zero in SQL.

Here is the SQL statement:

select Distributor.Name, sum(AllocationFund.Amount) as Allocaiton,
round(sum(AllocationFund.Amount), 0) as RequestAmount,
round(sum(AllocationFund.Amount) - sum(Request.Amount), 0) as Balance
from Distributor inner join AllocationFund
on Distributor.DistributorID = AllocationFund.DistributorID
left outer join Request
on AllocationFund.AllocTypeID = Request.AllocTypeID
and Distributor.DistributorID = Request.distributorID
group by Distributor.Name
order by Distributor.Name

Data for Distributor1 (Result)
Allocation Fund 1 = 45000
Allocation Fund 2 = 3000
Allocation Fund 3 = 1875

Request Amount for Fund 1 = 684.76
Request Amount for Fund 1 = 29600
Request Amount for Fund 2 = 1984.85
Request Amount for Fund 2 = 11998.52
Request Amount for Fund 2 = 444.58
Request Amount for Fund 2 = 375.14

The Result should be: Name = Distributor1
Allocation = 49875 (45000 + 3000 + 1875)
RequestAmount = 45088 (684.76 + 29600 + 1984.85 + 11998.52 + 444.58 + 375.14)
Balance = 4787 (49875 - 45088)

But I got the following result:
Result: Name = Distributor1
Allocation = 103875 (45000 * 2 + 3000 * 4 + 1875)
RequestAmount = 45090.0000
Balance = 58790.0000

How can I get the correct result?

Thanks.
DanYeung