Hi, 
From a US perspective - 
Typically, the advance invoice would be seen as a non-tax event.   The gross amount of the customer order would be the value for the advanced invoice.   Not Advance invoice for 100 + 7 Tax. 
The advanced invoice would typically be 107 with no tax.   If we charge tax in on the advance invoice, then tax reporting would include the advance invoice as a sale - it’s not a sale. It’s simply a request for a down payment (deposit). 
IFS has a setting - as to what values drive the advance invoice.  I’m late for a meeting - else I would include a print screen. Sorry. 
Best regards, 
 
                
     
                                    
            Hi, 
Did you find the setting that sets the basis for the advance invoice?   If not, later today, I can send print screen. I’m in another meeting now. 
Best regards
                
     
                                    
            Hi Thomas,
I am also having the similar issue where the CO is having multiple lines with different tax rates.
Line 1 - 0 Tax
Line 2 - 20% Tax
Line 3 - 0 Tax
 
While creating the CO Advance invoice it calculates 20% for line 1 & 2 as well.
 
Coild you please shate the scteenshot to check the setting.
 
Thanx in advance
 
RR