1) Ukraine has a long, epic history of governmental corruption - both sides of the house have been concerned about it in pre war days.
2) Biden is known for selling US foreign policy to foreign countries for personal gain (through himself and his family) and one of those countries in the past was Ukraine (see Burisma).
3) When aid began, Dems in the congress and Biden refused to allow aid to Ukraine to have oversight to assure it was all used as was being represented.
I do not know of a smoking gun that proves it, but it is not outlandish to be concerned about it, and it would not be surprising given the history mentioned.
Putin is a monster that needs to be opposed, but that does not preclude corrupt politicians from making a profit on it at the same time. The two things are not mutually exclusive, and in fact, I think that it is likely particularly because of #3. I have said this long ago myself….I want oversight of the money….you should too….so that it is ALL going to fight Putin, not just part of it.