Nedarim 84 - What's allowed when everything is forbidden
As we have seen, a husband has a veto power over the vows of his wife - but this is only if it concerns him in some way. For example, if she vows not to benefit from any human - he cannot annul that, because she can still avail herself of "owner-less" produce, which includes fallen grapes, forgotten sheaves, and the corners of the fields. All these are left by the field owner for the poor, in the "owner-less" state.
Why did this ruling leave out the tithe left for the poor? Do people leave over this 10% tithe at all? They do - because anyway they have an easy loophole to keep it: just declare all your possessions owner-less, then you are considered poor and can appropriate the poor men's tithe, and then reclaim your possessions. Thus those who want to be good and leave the tithe, can be relied upon. Another view is that they can't avoid the tithe easily, since they are afraid that someone will claim their owner-less possessions. Regardless, there is a group of people who does leave the poor man's tithe.
The question then is back, why not the tithe? - That is because he distributes it in his gates to whomever he wants , thus she is benefiting from his good will, which she forbade to herself. If he distributes it on his granary by leaving it over for all , then she would be able to take even that.
Art: Harvesting the Sheaves by Sandor Nagy