Just tried the same thing on demo server and its working fine.
Please reconfirm and provide steps to reproduce the issue.

http://latest.astppbilling.org/

Hi [~smrdoshi]

I just checked again current v3.6 and I get the same thing.

Steps to reproduce:
Set post-paid customer account to credit limit 10, set customer balance to 20 (over credit limit), try to create DID -> Enter DID number, select provider -> select customer account, click save. I get red warning triangle, with mouseover as shown. Setting Setup Fee & Monthly Fee to zero makes no difference, it still refuses.

I tried to test against http://latest.astppbilling.org/ but currently it's returning the PHP contents of index.php instead of running the PHP and returning output. Is that the demo server you mean?

Please can you confirm - for post-paid customers a positive balance is actually a debt (invoiced but not yet paid)?

http://latest.astppbilling.org/ is back now - so tested on there... Create post-paid customer, set credit limit to 20, create DID with setup cost of 20 and assign to customer - this works, and customer is now at credit limit. Change customer credit limit to 10 (to emulate CDR being invoiced to customer, taking them over credit limit). Now try to assign them a DID with zero setup and monthly costs and it fails as above.

Tried again with the steps which you provided and it worked.