How can we reconcile the hadith "The martyr is forgiven all sins except debt" with the hadith about the man who was killed at Khaybar and whom the Prophet saw in the Fire because of his misappropriation of war spoils, knowing that misappropriation of war spoils is a sin other than debt?

Martyrdom does not expiate the rights of human beings. The Hadith (definition), "The martyr is forgiven all sins except debt," includes all human rights. Debt and ghulul (stealing from the spoils of war before their division) are human rights, and they are not forgiven by martyrdom. Ghulul prevents the martyr from having his sins forgiven and may deprive him of the high status of martyrdom.