The transactions cannot simply be removed as they have gone through the process of matching and approving. They must be posted and credited in Dimensions.
To resolve this - our support team will need to investigate, run scripts or liaise with the hosting or development teams. Any Issues with contacting us, please see the following Get support in Dimensions.
Support will set the transaction to a state where it can be processed. You will then need to post and credit the transaction in Dimensions.