This is to do with offer and acceptance.
I think it would be useful if I explained how contract/agreement is formed. The words contract and agreement are interchangeable.
For there to be a contract there needs to be:
1 an offer and
2 an acceptance and
An offer can be accepted at any time until it is withdrawn.
An acceptance has to be unqualified and without conditions or extra stipulations otherwise it takes place as a counter offer which is then subject to acceptance by the original offering party.
Acceptance doesn’t have to be verbal, it can be by conduct.
Consideration doesn’t need to be financial, it just needs to be something of value passing from one person to another although the value can be nominal such as a peppercorn. If any agreement is by deed, then there is no need for any consideration although the majority of deeds do specify consideration.
A person cannot accept by silence. So you can’t say I will give you 5 pounds and if I do not hear from you by 5 PM then I will assume we have a deal. Caselaw is Felthouse v Bindley.
A person can accept by their actions if the actions are consistent with acceptance. For example, if you say that I will sell you this electric drill for GBP10 and you take the drill away and treated as your own and use it, it would be possible to argue that you had accepted by your actions.
Goods for sale in a shop are not on offer. They are an invitation to treat (an invitation to make an offer). The person wishing to buy makes an offer to the shop and the shop decides whether to accept or not. For that reason (there is already case law) that if something is marked at a particular price (even if marked by mistake) the seller does not have to accept that offer. There is additional case law which says that if there is an error which is a substantial error, even if they have accepted the money (the case law applies to Internet sale purchases) they do not have to complete at that price. The case law mentioned was for a television which should have been £600 which was advertised at £60 (or suchlike) and thousands of people bought the item in a few minutes. It went to court and the buyers lost.
So if somebody said that they would settle for GBP1000 and you went back and said okay that’s fine.
There is an agreement and they are bound by it and if they decided to take you to court for the balance, that is your defence.
If on the other hand they said that they would settle for GBP1000 and you said that’s okay provided you can pay it on 2 January, that is a counter offer.