I think I understand the confusion of the person asking the question. The so-called impoundment act, with the full name of Congressional Budget and Impoundment Control Act of 1974 was intended to prevent the president from withholding funds from programs he does not support even though Congress has approved funds for that program. For example, funds that are set for the EPA must be released to the EPA to be committed. The point that needs to be retained from this point is that those funds are available to be committed but do not need to be committed. Just like if your dad gives you $10 to buy movie tickets and you spend $8, you are not required to shred the remaining $2 or face prosecution.
The connection with the Ukraine controversy is that some budget office staffers quit over the feeling that it was inappropriate to withhold funds and cited this rule as the reason they quit. They are completely incorrect that this rule applies. They have every reason to feel what they want and quit for any reason but thinking that this rule applies is completely wrong.
The problem with this rule is 2-fold.
The congressional budget does not and cannot include specific foreign policy requirements. This is a power of the president described in the constitution and even though Congress can attempt to usurp any presidential power, it is up to the executive branch to keep that in check. The budget has a line for foreign aid and it is up to the president to spend it or not.
The rule states that the funds need to be made available for obligations, it does not require the president to spend any of the funds. So there is an account for foreign aid and it is up to the president to do anything he wants.
There is enormous examples of presidents withholding or over extending the budget line for foreign aid since this rule was enacted.
If you just take the year by year summary of appropriation vs disbursement (source explorer.usaid.gov) you can see that spending is often less than appropriations and sometimes higher (2015, 2016).
Now the devil is in the details, never has there been a year without appropriations but what was the money spent on. During the Obama administration, there was a question of lethal aid or not, Trump did not get around to making a change until late 2017 and begin sending lethal aid, but the dollars did make it to Ukraine every year in one way or the other.
This whole discussion is entirely academic since there is still no proof that any aid had been withheld for any extensive period. Certainly not for a year, the numbers do not lie.
The question of aid is so convoluted that it does not make sense to question whether a president can or cannot withhold aid as in all aid. The president can play with some disbursement but in a practical sense, no one agency can stop everything at once since there are so many disbursement from so many agencies. For example just Ukraine in 2019 has as many as 303 sources of disbursements (Source: explorer.usaid.gov):