This same topic has been discussed numerous times on other threads.
I found the last time this happened to me (it's happened to me at least twice IIRC), contacting the Savings Bonds customer service people by e-mail apparently did a world of good. (I had already called Amex and argued with them at length, to no avail. Amex pointed fingers at the US Treasury.) In other words, once the actual purchase went through, the "hold" against my credit limit could be released, as long as the US Treasury Savings Bonds people approved same.
Kathy