I don’t think it’s possible to do this manually (though if you open a new chat with COps they may be able to help). Otherwise, when Steam come to collect the money in a few days the £40 should then leave your account.
From when I did a reversal on Google Domains a few days ago, it did state that the merchant can still collect the money if necessary, so I believe they’ll just take the payment as normal.
Screenshot with the wording below;