You may be an oracle on transfers on this forum, however, I do have a certain degree of understanding of transfer workings. Granted, there might be other options available for United. Likewise, there are no guarantees there are no alternative clubs willing to pay Porto more than United for Telles. This is how an open and free market works.
The only way United end up paying less is if Telles insists its only United he'd move to this window. Porto would then have to evaluate options available to themselves. Either to release him for a substantially lower fee than they were expecting. Keep him and hope another suitor comes in for his signature near the end of the transfer window or in January. Alternatively, he relents and signs a new contract. The worst-case scenario for Porto is leaves for free in the summer.