sql select problem - subquery returned too many values

I have 2 tables Customers and Transactions.
Customers has a ref for each customer called InternalRef. We have a Supporter that also has an InternalRef as well as a SupportersRef which is linked back to Customers.InternalRef. That means they support that Customer.
A transaction can be made by both a Customer and a Supporter. If a Customer makes a transaction entry then their InternalRef is stored in the Transactions table. If a Supporter makes a Transaction entry then the supporters internal ref from the customers table is stored as a SupportersRef in the transactions table as well as the InternalRef of the Customer who they are supporting.

In both cases the customers InternalRef is stored in the Transactions table as InternalRef. Only if a supporter makes a transaction do we have an entry in Transactions.SupportersRef.

Customers Table:
InternalRef  Name SupportersRef   CustomerType
123             Jim    null                   Customer
124             Fred  123                   Supporter

Transactions Table:
InternalRef   SupportersRef   Amount
123              null                  10
123              124                  20

Customer Jim would see 2 transactions and supporter Fred would only see 1 transaction (the one with supportersref 124 in the Transactions table).

I want to return the 2 transaction for Jim - 1 with the name Jim and 1 with the name Fred, so that the customer knows where the transactions originated from.

Sample Jim sees the following:
Name       Amount  
Jim          10
Fred        20

Fred sees the following:
Name       Amount
Jim           20

I need the solution for Jim only as Fred is working fine.

Sample Query that works for Supporter Fred:

IF @CustomerType = 'Supporter'
BEGIN
select Transactions.InternalRef,Transactions.TransactionDate,Transactions.Amount,   Transactions.Description,Customers.SchoolOrSupporterName, Customers.SupportersRef, Customers.CustomerID
  from Transactions
  inner join Customers on Transactions.InternalRef = Customers.InternalRef
  where Transactions.SupportersRef = @Ref
  order by Transactions.TransactionDate DESC
END

I tried the following but did not work and gave the error subquery returned too many values:

IF @CustomerType = 'Customer'
BEGIN
select transactions.*, customers.*, IF (Transactions.SupportersRef IS NULL) BEGIN (select Customers.SchoolOrSupporterName from Customers inner join Customers on Transactions.InternalRef = Customers.InternalRef where Transactions.InternalRef = @Ref) END ELSE BEGIN (select c.SchoolOrSupporterName from Customers c inner join Customers on Transactions.SupportersRef = c.InternalRef where Transactions.InternalRef = @Ref) END AS CustomerOrSupporterName
..............
END