Hi John,
I think the problem stems from the fact that
acheter quelque chose à quelqu'un
can mean both
to buy someone something ( to make a gift)
and also
to buy from someone.( as a transaction).
In this case as it a 'commerçant' we know that the person wants the same joint of beef that he/she bought from him (= that he sold her) last time.
Take a look at these examples -
Il ne faut jamais acheter à quelqu'un dans la rue = You must never buy from someone in the street
Pour Noël, je leur ai acheté un abonnement à Paris Match = For Christmas I bought them a Paris Match subscription
J'ai acheté une parcelle de terrain à mon voisin -----> Je lui ai acheté une parcelle de terrain = I have bought a piece of land from my neighbour ----> I have bought a piece of land from him
It looks confusing but the context will determine whether it is a gift or a transaction.
The pronouns you will use will be indirect object pronouns to replace the preposition à.
Hope this helps!
