Remember back in S2 when Mike did the heist on Salamanca's truck shipment and left the driver tied up on the side of the road in the desert? A Good Samaritan came along and freed him, and as a reward was murdered by Salamanca. So, last week after the group therapy session, Anita told Mike how much it bothered her that she never got any closure from the death of her husband in the line of duty years ago because they never found the body. As a result of her story, Mike felt guilty about this other guy getting murdered as collateral damage from the heist Mike set up, and his loved ones (hence the shot of the wedding ring on the hand) not getting any closure either, so he asked Nacho for a favor about where the body was buried, and it turned out to be in pretty much the same spot where the heist went down and the Samaritan showed up near the billboard.
Apparently squaring up that unfinished business also inspired Mike to realize that he had too much money stashed away and no way to spend it on or give it to his granddaughter for her future, which is why, despite his misgivings, he is now going to go into a business arrangement with Gus Fring to launder the money in exchange for his services.
So, in a way, both those stories addressed the "Debt" theme you mentioned above. Mike felt a debt to the family of the dead Samaritan for getting him killed and not giving them closure, a debt to his daughter-in-law and granddaughter for indirectly causing the death of his son, and as a result of trying to pay off both of those, has now fatefully indebted himself in the employee of Gus Fring.