SAP Note 10885 - Changing terms of payment during posting.

Symptom

Terms of payment changed during ONLINE posting (FB01, SAPMF05A).
The baseline date for payment does not function "correctly".

Cause and prerequisites

The baseline date for payment is only suggested once; as soon
as the terms of payment are changed, the calculation of the new
baseline date for payment always includes the current contents
of field BSEG-ZFBDT. The default in the second term of payment
therefore has no effect. Only the calculation part and the
contents of field BSEG-ZFBDT influence the newly calculated
date.

Solution
When you overwrite the terms of payment, always check whether
you also have to overwrite field BSEG-ZFBDT.

Key word: Baseline date