Hi Jobish,
It's weird but I tried the below method and it works 
Here's what you can try in case you do not want to enter the bank details for payment method 'C'..
Tcode FBZP - > payment methods by country - > select payment method as 'C' and make below changes (make a note of these changes as you will have to undo them after saving the table):
a. Payment method by classification - select bank transfer
b. Reqd master record specifications - check the bank details options and select the options which you want to be entered (a/c no required, IBAN rqd, SWIFT code rqd)
Save the table.
PA30- > Edit any record once again and save it.
Again go to FBZP and undo the above changes, i.e, revert to the original entries and again save the table.
Try editing the record now in PA30 and it allows to save. This is the second weird workaround that I have come across and still not sure why
After above workaround, I didn't get any error while maintaining the record, for this or any pernr.
See if it works.
Regards
Neha