Power of Attorney is a legal document made for authorising another person to do a particular act or certain acts as prescribed in the power of attorney document. GPA or General Power of Attorney gives more general authorisation for doing all that is needed in relation to the acts prescribed( say carrying out the business, trading the properties , managing the assets etc. . GPA is given to a person so that he or she is not finding any restrictions to carry out the act on behalf of the principal (person who gave GPA). The power of attorney holder can sign on all places on behalf of the principal as POA,and it becomes valid.
Sale deed is the legal document executed by the seller , and narrates a sale of a particular property, owned by the seller, and transferring the ownership and possession to the buyer. The document describes the name and address of the seller and buyer, the full details of the property and the total amount of sale price and how it is given and received. The deed is made on stamp paper of requisite value or stamped for the value and registered with the relevant registrar office.
If the owner seller has made a POA in favour of another person, that Power Of Attorney holder will sign the sale deed on behalf of the owner seller signing as POA holder for (name of seller).
Power Of Attorney also has to be executed and registered duly a per legal provisions. Both POA giver and POA holder have to be properly identified and witnessed,.
In the present case as narrated, it may be preferable that the neighbour lady herself executes the sale document if she is normal healthy and can visit the registrar office. Your husband can give her all other help from finding a buyer till finalising a deed. However if she is too weak and there is great mutual faith between both your husband and the lady, then probably your husband can accept the POA and do the needful. Then you must take care to avoid misunderstanding and avoidable legal issues from her close relatives.