Hi there and thank you for your question,
As I understand your question, the company whose account has been blocked has been given a line of credit from the supplier? And the supplier has blocked that line of credit because a different company (owned by the same people) is in arrears?
YES, the supplier is perfectly entitled to do so.
There is no right in SA for somebody to give another person credit, or an account. That is a privilege.
That means that the supplier is perfectly within their rights to block/cancel the account for whatever reason it wants. ANY reason. Maybe they don't like the fact that you drive a Merc, that's a good enough reason.
My advice, get the other company to settle it's debts so that the supplier will release the account for the first company.
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Good luck and best regards,
Please note: This answer does not create an attorney-client relationship. Please consult a local attorney in person for legal advice. This information is being provided so you can better discuss legal matters with your attorney.