VBA Code To Check That Invoice Number Has Not Already Been Used

I use a userform to make sales data entries into a database. The userform contains textboxes for:

Invoice No.Invoice DateGrossVatNet The Invoice No. is the first entry box on the userform and it is important that users enter the correct format (which is alphanumeric).

It is also important that a checking is carried out to ensure that the Invoice Number being entered has not already been used.

I use the following code to ensure that the numbers are entered in the desired format:

Code:
Private Sub txtInv_Exit(ByVal Cancel As MSForms.ReturnBoolean)
If txtInv.Value = vbNullString Then Exit Sub
If (Not UCase(Me.txtInv.Value) Like "ST###") And (Not UCase(Me.txtInv.Value) Like "ST####") Then
MsgBox "Non Valid Invoice Number.”
Cancel = True
End If
End Sub

And this code is pasted lower down the VB editor window. And it seems to be working fine!

For the checking to ensure that the invoice number being typed into the textbox has not already been used, I use the following two codes combined together:

a) to check for invoice No.:

Code:
check for Invoice Number

If Trim(Me.txtInv.Value) = "" Then
Me.txtInv.SetFocus
MsgBox "Please Enter Invoice No."
Exit Sub
End If

b) And to it I add the following additional code for CHECKING that the no. does not already exist in the database (the invoice nos are located in Column A of the database):

Code:
Dim x
X= Application.Match(Me.txtInv.Value, Columns(1),0)
If Not IsError(x) Then
MsgBox Me.txtInv.Value & "Invoice Number Is Already Used"
Me.txtInv.SetFocus
Cancel = True
Exit Sub
End If

My problem with the second set of codes in (a) & (b) (for the Checking of already existing No.) is that the code allows the user to key in the Invoice No. and go through filling in the other textboxes. It is only after clicking the ‘Add’ button that a msgbox comes up to indicate that the ‘invoice no. is already used’.

I need a tweaking of the codes so that the moment an invoice no. that has already been used is typed into the textbox and the TAB button pressed, a msgbox will show up at that stage to indicate that the ‘Invoice no. is already used’. This will reduce time and avoid filling in the other textboxes and clicking the ‘Add’ button before the checking is done.