Tax Amount is normally deductible which means tax is not considered as the expense and it is created as Balance Sheet account. Budget control works on actual expense/asset value, not on deductible tax part.
If in your case, tax is non deductible which means your customer wants to include tax to expense/ asset value, you have to create Tax Condition as non deductible and system will consider this value in budget.
Please check if your tax is deductible or non deductible and act accordingly.
Thanks for reply. This in case of Asset PO where we are using IO & tax. If this is standard behavior do you have any note to show the client.