Praise be to Allaah.
Firstly, it is
permissible to buy chicken by weight. This is the basic principle and we
do not know of any evidence (daleel) to the contrary.
Secondly, it was
reported that the Messenger of Allaah
(peace and blessings of Allaah
be upon him) said: “Whatever intoxicates in large quantities, a small
amount of it is haraam.” (Narrated by Ahmad in al-Musnad, 2/91, 167,
179; 3/343). If the vinegar would cause intoxication in large amounts,
then a little of it is haraam, and it comes under the same ruling as wine.
If a large amount of it does not cause intoxication, then there is no
reason not to sell it, buy it or drink it.