You cannot switch plants: Different decimal points (see long text)

What causes this issue?

During data entry, you have entered or changed a plant. You have also,
either previously or in the same step, entered at least one external
plant.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

<DS:DE.EXBWR>EXBWR</> (Externally entered posting amount in local
currency)
<DS:DE.EXVKW>EXVKW</> (Externally entered sales value in local currency)
After this entry, the system determines which currency is the default in
the plant. For each currency, you can make settings in Customizing under
<LS>Set decimal places for currencies</> to specify the number of
decimal places. This means that the currency and number of decimal
places can change depending on the plant:
The currency in plant &V1& has &V2& decimal places.
The currency in plant &V3& has &V4& decimal places.
(If you have not entered a plant, the system always uses two decimal
places).
Switching the decimal places would cause the following situation to
occur:
Example 1)
You have entered an external posting amount in local currency for '1234'
for plant '0001' (currency: Euro, decimal places: 2). After conversion,
the amount is displayed as follows: <ZH>1.234,00</>.
You then change the plant to '0002' (currency: Japanese Yen, decimal
places: 0). Now the external amount is converted according to the new
currency, so that the correct value (with regard to contents) would not
be displayed: <ZH>123.400</>.
Example 2)
The default values for transaction <ZH>MIGO</> do not contain a default
for the 'Plant' field. This means the system cannot determine a
currency. In this case, the system always uses two decimal places. If
you now enter an external posting amount in local currency of '1234'
with plant '0002' (currency: Japanese Yen, decimal places: 0), the
system uses two decimal places instead of 0. For this reason, the
external posting amount '1234' would not be converted into the correct
value (with regard to contents) <ZH>123.400</>.

How to fix this error?

To avoid this situation, proceed as follows:
For example 1)
You can only switch plants, where the number of decimal places would
change, if you have not yet entered an external value. Therefore, you
should first enter the correct plant before you enter an external value.
If you have already entered an external value, you must delete this
value before you switch plants.
For example 2)
Alternative 1:
First, enter the plant and confirm with <LS>Continue</>. You can now
enter the external value.
Alternative 2:
If you post to the plant with the different number of decimal places
(not two), then the default values (<LS>Settings -> Default Values</>)
should contain this plant as a default. This means that you can
immediately enter the external value.

Error message extract from SAP system. Copyright SAP SE.