The head of the executive agency is bound by law to follow an executive order. However, the funds to do so must come out of the agencies existing budget. The executive agencies must still follow budget rules and policies for expending funds. So Congress can effectively squash an executive order by not funding it. They can also pass legislation which conflicts with the executive order. To keep the executive order in effect the President would have to veto the legislation and hope that Congress doesn't have enough votes to override the veto.
Have a nice day at the range
NRA Life Member