Version:           4.6.0 (using KDE 4.7.2) 
OS:                Linux

If a Payee has transactions in different currencies (say GBP and EUR), they are shown on the "Payees" screen each in their original currency (why not), and the "Balance" field at the bottom of the screen is the sum of all these transactions in their original currency. The latter is clearly incorrect, as it's adding apples and oranges,

This is best examplified by creating a single transaction, a transfer between two accounts in two different currencies (e.g. GBP and EUR) for a new Payee. Say I transfer GBP 100.00 from the account in GBP to the account in EUR, which becomes EUR 114.56 at the other end. The list of Transactions under "Payees" for that Payee shows two transactions: a Deposit of 114.56 (EUR) and a Payment of 100.00 (GBP). This is not incorrect per se, although it could be misleading. What is clearly incorrect is that the "Balance" at the bottom is 14.56. This is incorrect as it's subtracting a GBP amount from a EUR amount. Given that these are the two sides of a single transaction, the Balance should be 0, which KMyMoney should arrive to by applying the Exchange Rate used when the transaction was created.

Reproducible: Always

Steps to Reproduce:
- Create a new account in GBP (or use an existing one)
- Create a new account in EUR (or use an existing one)
- Create a transaction from the account in GBP:
   . Pay to: <A new Payee> (for clarity's sake)
   . Transfer to: <The account in EUR>
   . Amount: 100.00
- <Enter>
- In the Exchange Rate/Price Editor window:
   . To amount: 114.56
   . Update Price history is ticked
- <OK>
- Go to <Payees>
- Select the new Payee
- Balance at the bottom is: 14.56

NB: I have checked by selecting  Tools > Prices... that the FX has been stored for that date: GBP -> EUR Price = 1.1456

Actual Results:  
Balance: 14.56

Expected Results:  
Balance: 0

My suggestion is that each individual transaction on the list under "Payees" should be converted into the Base Currency. This should be done using the stored Exchange Rates/Prices. If no stored Exchange Rate/Price is found for this date, then the most recent one going backwards into the past from the transaction date should be used. If none is found, then a default Exchange Rate of 1 should be used. This situation would result in the same result as above (Balance: 14.56), but this would be justified by the absence of any stored FX value for that pair of currencies.
The Balance field can then safely be calculated by adding/subtracting the amounts for all these transactions.

OS: Linux (i686) release 3.0.0-12-generic
Compiler: gcc