Congress can impose sanctions-- the sanctions against North Korea are an example. The President can veto the sanctions, however. So the question is if the President does not veto this bill or if the veto is overridden by Congress does the restriction on the President changing the sanctions stand as constitutional? It would seem reasonable that it would stand since the President's veto power was overridden by Congress (or waived by the President). Powers over foreign affairs are divided between Congress and the President and also shared and I believe sanctions are an area they are shared-- via the veto power and via the requirement that the Senate approve certain Presidential measures.