The executive has discretion in how funds are disbursed, but they have to fulfill all the obligations laid out by Congress. Impoundment is expressly illegal, not just due to Article 1, but also the Impoundment Act to avoid any ambiguity. The Dept of Education, for example, is created by act of congress and has a list of obligations in the congressional budget and the president has no authority to deny that. They have discretion in terms of how it is fulfilled and who gets paid when, but they are assuredly not allowed to just cancel programs or agencies that explicitly funded by congress.