Hi Axel,
Message M8 534 says that the invoice is not in balance - debits and credits don't total to zero. It is normally impossible to save such invoice, unless you use the tolerance BD "Form small differences automatically". But this one is really intended for rounding differences, not more than that.
In the upper right corner of MIRO you will see the current balance of the invoice. If it isn't zero (and you haven't defined the tolerance BD) then you will not be able to post - please adjust your amounts (probably item amounts).
Note that the tolerance BD works together with the process DIF in the account determination (trs OBYC) - there you define which account the small balances shall be posted to.
The price tolerance will be checked as soon as you change the suggested item amount - the system will check the entered amount against the calculated one. But this is of course if you have defined the price tolerance (PP or PS).
BR
Raf