Bhisma wanted to repay the debt, He owned to the Hastinapura and Kuru Dynasty
Contrary to the Popular belief that he was bound by Promises and some sort of boon is a false fabricated story.
When Yudhistra approached Bhisma for blessings and Permission before the war, This is what He says:
"Bhishma said, 'If, O lord of the earth, thou hadst not, in this battle come to me thus, I would have, O great king, cursed thee, O Bharata, for bringing about thy defeat. I am gratified (with thee), O son. Do battle, and obtain victory, O son of Pandu, What else may be desired by thee, obtain thou in battle. Solicit also the boon, O son of Pritha, which thou desirest to have from us. If it happens so, O great king, then defeat will not be thine. A man is the slave of wealth, but wealth is no one's slave. This is very true, O king. I have been bound by the Kauravas with (their) wealth. It is for this, O son of Kuru's race, that like a eunuch I am uttering these words, viz.,--Bound I am by the Kauravas with wealth. Battle excepted, what dost thou desire?
Bhisma explains that he is bound by the wealth of Kuru Kingdom. Bhisma acknowledges the power of material obligation and patronage.
The Kaurava kingdom has supported and maintained him. He has accepted their patronage and therefore considers himself obligated to serve them.
This is what Dhritarastra says to Sanjay:
Bhishma, that regenerate Drona, and Kripa the son of Saradwat,--these are as much acquainted as I myself with the energy of the intelligent Bhima. Acquainted with the practice of those that are noble, and desirous of death in battle, these bulls among men will take their stand in the van of our army. O Sanjaya, the little acceptable services that we have been able to do unto these three venerable ones, will certainly be repaid by them owing to their own noble dispositions.
He tells that They have done venerable warriors certain favors and provided them with support. Because they are noble and grateful men, they will repay those favors by fighting for us.
My Answer is based on Mahabharat Translation by Kisari Mohan Ganguli (KMG).