The only sources are Greek or western. We have no Indian sources of the battle or the aftermath.

There was an Indo-Greek culture and kingdom. It is apparent at the very least some understanding was reached.

Western sources say that Alexander made Porus a Client King and enlarged his lands.

This seems the basis for the argument. If he lost why leave him as a king and give him more lands.

That seems to be the big sticking point in modern arguments.

It was not the end of the campaign of course as the army continued south securing borders before returning west.

There would be little need of that had he met with defeat. Obviously they were cementing gains not abandoning the area to a victorious enemy or doubtful friend.