Try this not 100% sure if it will work. Replace the Invoice No into a Temp field. Then choose Invoice No again but it will be a CALC like this
PAD(Temp001$,7,$00$)
Change Temp001$ to the correct number if needed. What this does is pads the 6 digit number to the right with the Null character
If this works and assuming you're doing in a test company do a cash receipt on the invoice, print the register, update it. Make sure it worked right.