According to the poem "We Wear the Mask" by Paul Laurence Dunbar, how do we pay off our debt to human guile?

The short answer is that paying our debt to human guile means that we "pay" for the "service" of hiding our true selves, which the mask provides. And who is it that we owe something to? Well, it's not a who, but a what: we owe something to human guile, or to the entire human race's tendency to use cunning or deceit for their own personal purposes.

This question refers to, quite possibly, the most...

The short answer is that paying our debt to human guile means that we "pay" for the "service" of hiding our true selves, which the mask provides. And who is it that we owe something to? Well, it's not a who, but a what: we owe something to human guile, or to the entire human race's tendency to use cunning or deceit for their own personal purposes.

This question refers to, quite possibly, the most difficult line of the poem. Let's look at it in context:

"We wear the mask that grins and lies,

It hides our cheeks and shades our eyes,—

This debt we pay to human guile;"

(For the purpose of understanding the question at hand, we can stop examining the line right there. The semicolon after "guile" shows us that there's a new, independent idea coming up, meaning that's also where this whole idea is finished being expressed.)

So what these lines are saying is: "We put on a fake face, which smiles and tells lies, and it keeps our real faces hidden. And because we are able to do this, we owe it all to our species's natural tendency to deceive others."

If you want a fuller answer that makes a logical leap from the text (an inference) rather than a factual explanation, you could venture to say that we pay off our debt to human guile by enduring the suffering that comes along with keeping ourselves hidden.

For some support for that idea, check out the final stanza of the poem. There, the speaker describes how the "we" in the poem have tortured souls, how we cry out to our god for relief, and how we see our earthly experience as "vile" and protracted (lasting a long time). These details could back you up if you needed to prove that the poem does hint that the "debt" is repaid through suffering.