Thank you for posting here in the Community, @tonibw.
I'll help you recreate the transfer transaction that you deleted accidentally.
If the transaction was from the bank feed, import the bank transaction manually. On the other hand, if manually entered, you can manually recreate the transfer.
Here's how:
Go to + New.
Click Transfer.
From the Transfer Funds From dropdown list, choose the bank account from which the money is coming.
From the Transfer Funds To dropdown list, select the bank account the money is going to.
In the Transfer Amount field, enter the amount to transfer.
Edit the Date field.
Enter a description of the transfer in the Memo field (optional).
Click Save and close or Save and new to complete the transfer.
Select Yes if you double-checked the date.
You can check this article for more information about transferring funds from one bank to another: Transfer funds between accounts.