Your dealer is practicing self preservation. As in, he's trying to blame GM when it really is just him being greedy.
HE further says MSRP is the lowest they would go, and it was not up to them, it was a decision by GM.
"Would" being the operative word there. "Could" would imply that they was nothing they could do about about it. "Would" says it was their decision.
Federal franchise laws prevent auto manufacturers from controlling prices at the franchise dealer level. They can suggest a price but they can not dictate the final sales price. Most states have laws in place that also prevent manufacturers from selling direct to consumers and also prevents them from owning franchises directly.