If you're IFR and not cleared a visual approach, are you even allowed to descend below the MSA until either established on the approach or flying a visual approach? ATC cannot give you permission to break the rules. Same with speed restricted by airspace classification (250 below FL100), you can ask all you want, but you're still not allowed to go faster.
But for enroute we're not allowed to give clearance below "minimum flight altitudes". And the responsibility lies with ATC during vectoring or rerouting where we take the aircraft off track.... generally.