斤: a bread unit and a traditional weight unit
Read 一斤 in context as a unit of sandwich bread or a traditional weight entry. The character 斤 alone does not supply one gram conversion for every item.
一斤 when buying bread
斤 is read きん. 食パンを一斤買い、半分は冷凍した。 says that someone bought one loaf unit of sandwich bread and froze half of it.
In this sentence, 一斤 gives the amount of bread purchased. 半分 then refers to half of that bread; it does not mean that two loaf units were bought.
斤 in an old ledger
古い帳簿には小麦一斤と記されていた。 says that an old ledger recorded one kin of wheat. Here, 斤 is a traditional unit used to record the wheat’s weight.
The bread sales unit and the traditional weight entry involve different items and conventions. The sentence does not state a conversion into grams.
Do not convert from the unit name alone
A label of 一斤 on bread does not establish that it weighs the same number of grams as 一斤 of wheat in an old ledger. First identify the item and the convention used for its label.
If a problem requires grams, you need the bread’s relevant labeling information or the applicable traditional weight standard. Choosing a conversion from the name 斤 alone adds information the sentence does not give.
Must 一斤 of bread and 一斤 of wheat in an old ledger weigh the same because the written unit is identical?
Show the answer and why
You cannot conclude that. Distinguish the bread sales unit from the traditional weight unit and check the conversion standard for each.