Can you go to the issuing bank's web site for the account it was assigned to, to see if it really exists on that account? It's a good idea to do that regularly in any case.

It could be a transaction that you entered manually, but when it imported from the bank it was a different amount.

It could be a transaction entered on the wrong account.  To check, go to all accounts and search for that amount.

It could be a transaction that just hasn't cleared yet.

It either hasn’t cleared yet or you entered it in the wrong account. Look at the all accounts view to see if it also exists in another account.

If your account reconciles and the transaction is so old you are sure it's not pending at your bank, then the transaction either is an error on your part when you entered (e.g wrong account) or it hasn't match with an imported transaction at the time so it's a duplicate of a transaction you already have (e.g. manual and imported transaction dates too far apart, payees and amount different if you made an error in the amount ...). In both cases, no matter the reason, it can be deleted.
It will increase the available in the corresponding category(ies) on the budget screen.

Another possibility, if it was a check, it may not have been presented for payment yet