When you say that Partner A was informed that he could not take any more money out of the credit line, assume that was communicated by Partner B to Partner A but not to the Bank? On that basis, here are the things I think you need to do.
First review the terms of the guarantee to make sure you are subrogated to the Bank's rights if the bank requires you to pay on the guarantee.
Second, notify the Bank that Partner B's business has no relationship to Partner A's business.
Third, notify the Bank that you are revoking any right the bank has under the guarantee to modify the terms of the underlying credit line or to make advances under the credit line or renew the credit line without Partner B's written approval. You should make sure that your guarantee will not cover any such changes made without Partner B's approval. If the Bank was aware that Partner A was not to increase the credit line, you should also state that to the Bank in writing.
As a guarantor you cannot revoke your guarantee after the bank has advanced funds. However, you can prevent your guarantee from applying to increases in or changes in the terms of the underlying debt being guaranteed without your express approval.
Had this been done at the time of the split, your guarantee would not cover the increased borrowing under the credit line.
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