Whether a lender can reduce the credit limit because of a downturn in the real estate market is debatable at best. The basis for reducing the lending limit is the "conumer's action or inaction" and not the actions of a third party. The commentary states:
5b(f)(3)(vi)… “A creditor may prohibit additional extensions of credit or reduce the credit limit in the circumstances specified in this section… A creditor may not take these actions under other circumstances, unless the creditor would be permitted to terminate the line and accelerate the balance as described in §226.5b(f)(2).
Paragraph 5b(f)(2)(iii)… 1. Impairment of security. A creditor may terminate a plan and accelerate the balance if the consumer's action or inaction adversely affects the creditor's security for the plan, or any right of the creditor in that security. Action or inaction by third parties does not, in itself, permit the creditor to terminate and accelerate.
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Merely my opinion...