Let get this right: You negotiated a MF few month back, but when you actually signed the papers they used the current one??
If that is what you mean, your negotiated MF should've been locked for 60 days (or 90 days in West Coast) when you got approved for the loan by BMWFS. So if you applied let's say in November and it was .00135, it should have been good for 60 or 90 days. But then January it's time to sign, and the MF is .00130, they should use .00130 because it's lower.
I'm no expert but that's how I understands it.