Budget calculation with explanation code S.

Hi everybody I have a doubt about budget calculation when tax explanation code (EXR1)  "S".

I have an account with a budget of $1000 and the following scenarios:

1. If I create an order with only one line of detail by an amount of $1000,  the order is not placed on hold.

2. If I create an order with two lines of detail by an amounts of $500 and $421 respectively,  the order is placed on hold although the amounts do not exceed the available budget. I thought that having explanation code "S" the system add the amount of tax to each line (in my case 16%) and for this reason the order was placed on hold.