Maybe I've missed the complexity of your account number,
Yet assuming your account number is in a column within your list which you are querying, why not do the following.
On your form you'll have a field where an account number is input into (in my test it was a simple number). i.e. New Account Number
You'll have a section, which has text in it i.e. Account number already exists, please choose another one.(Make if very obvious for your end users).
Attached to the section above is another field (Existing Account Number), which is not on the form, but used for "math". default value of 0
You could use a button (next to or underneath your New Account Number field) with several rules on it
1. Reset field Existing Account Number to 0
2. Quwery your sp list
3. Condition - New AccountNumber = Account Number
Set Field Value Existing Account Number = 1
Then on your section (Existing Account Number) have a formatting rule to hide the section if Existing Account Number IS NOT EQUAL to 1.
On you button with the rule, make sure that if rule 3 is met, any further rules are not allowed to run.
You could hide a separate submit button until the account number has be checked and verified.
It doesn't have to be a section that hides, you could switch to a view to make it very obvious. with a back button (switch view) to the previous view.