Posting in foreign currency

I would like to be able to enter the exchange rate or the amount of the debit in the base currency in the entry screen.
This option is available for transfer Transaction, but not for normal Transaction.
See also: #4450

Reason: the valuation of the individual Transaction currently carried out according to an algorithm unknown to the user and results in differences in the accounting.

Example:
My base currency is CHF
My $ account has a balance of 1000 $
I entered the exchange rate on Nov. 1 $ = 0.91
I pay with Paypal on 10. Nov 100$ at the rate of 0.90 = CHF 90.00
I entered the exchange rate on Nov 15 $ = 0.89
I pay on Nov 24th with Paypal 100$ at the rate of 0.88 = CHF 88.00

I have paid a total of CHF 178.00. (sorry, my example could be better, then there would also be a difference here)

Here is the evaluation from MMEX:

It is calculated at "Gifts" with the rate of Nov 1, 0.91, as the sum of the "Expenses" (random correct) is calculated = ?

I have now downloaded the current rate from Nov. 25th and the values are completely different.
The "Expenses" are shown correctly, but the "Gifts" are now valued at the last known rate and therefore now cost less.

I would like to point out that this type of treatment of foreign currencies does not make sense and hope that my proposed changes will be successful.