Hi @Kate
In this case, you would create a purchase invoice for the balance (in this case, £100) and note what nominal ledgers it affects. This should be dated before the date you started using QuickFile.
You would then adjust your opening balance journal to account for this invoice, otherwise the amount gets counted twice.
Then, when the supplier pays, you can tag it to the outstanding invoice.
There’s an example in this post, which may help: Struggling to get started