Is it permissible to resort to trickery to take a portion of the Value Added Tax (VAT) funds due to me and pay the other portion to the tax authorities, if purchases were not made with tax invoices, even though the contract stipulates otherwise?

The VAT amount must be returned to the company that provided it, unless it has been paid, because it was not given as an ownership transfer, a gift, or part of the price. Rather, it was given on the basis that you would pay it. Therefore, you are considered an agent for the company in delivering the tax, and if you have not paid it or have paid only a portion of it, the remainder must be returned to your principal.