The argument from Khaybar sounds impressive at first, but it does not prove what you are claiming, Alalubosa. 

After the conquest of Khaybar, the Prophet ﷺ allowed the Jews to remain and cultivate the land in return for an agreed share of its produce:

أَقَرَّهُمْ رَسُولُ اللَّهِ ﷺ بِهَا عَلَى أَنْ يَعْمَلُوهَا، وَلَهُمْ شَطْرُ مَا يَخْرُجُ مِنْهَا

“The Messenger of Allah ﷺ left them there on the condition that they would cultivate it, and they would have half of what it produced.”

So, where exactly is the partnership with ribā in this narration?

The Prophet ﷺ did not say: “I shall pool my lawful capital with your ribā-based wealth and we shall establish a joint business.” Rather, the Jews were retained as cultivators of the land, receiving an agreed share of its agricultural produce.

Yes, the Jews were known to engage in prohibited dealings, including ribā. But the mere fact that a person engages in some ḥarām transactions does not make every transaction with that person ḥarām. The relevant question is: What is the particular transaction, what property is involved, and what activity are the parties actually agreeing to undertake?

This is where the argument makes an unjustified leap:

“The Prophet ﷺ dealt with Jews who practised ribā; therefore, it is permissible to enter into business partnerships with people whose capital or earnings involve ribā.”

No. That conclusion does not follow from the evidence, Alalubosa. You really need to be very careful with such a careless generalization especially when you are trying to impugn the personality of the Noblest of Mankind - sallallaahu alayhi wa sallam. 

Khaybar is evidence that the Prophet ﷺ could engage in a lawful economic arrangement with people who themselves engaged in unlawful dealings. It is not evidence that he ﷺ participated in their ribā, incorporated their ribā-derived wealth into his capital, or approved a partnership whose capital or business activity was ḥarām. Get it right, Alalubosa, you really need to sit and ponder very well. 

This distinction is not a technicality. It is the very point upon which the argument stands or falls:

Dealing with a person who has ḥarām earnings is not equal to partnering with him in ḥarām wealth or activity.

Therefore, whoever uses Khaybar to establish the blanket principle that “it is permissible to enter into partnership with someone known to have ribā earnings” needs to produce evidence for that specific claim. Khaybar does not establish it. Alalubosa therefore needs to dig his land further for a non existent proof. 

At most, Khaybar establishes the narrower principle that a person's involvement in some prohibited transactions does not automatically invalidate every otherwise lawful transaction he has with a Muslim.

And that is a very different proposition.