Fantasy D&D-like world. In a kingdom we have a rule, that when king starts a war, from every noble family one person has to join the army (nobles take at least squad leader positions, townsfolk and peasants get lower positions, everyone can get promotion if they are skilled). Sometimes nobles try to bypass it by adopting someone just for him to go fight instead of their firstborn, but it's risky as adopted person can, but not must, waive inheritance rights after coming back from war (they can also be disinherited, but this creates additional problems).
Our nobleman has a problem - he is not able to fight (illness) and his only able child is his firstborn daughter. He wants to send bodyguard with her, but it's illegal. He cannot also bribe her way to safety, by not knowing who to bribe until it's too late.
His solution: he has a friend, old war veteran and known general, who has no children and agreed to adopt someone who will be told to protect nobleman's daughter, but old general made it clear, that his adopted son must meet some steep expectations.
They found the right candidate... how to convince him? Old veteran is not rich nor influential or powerful, so inheritance from him would give our candidate title of nobility, good reputation and maybe nice house, but no servants or land. They don't really have time for elaborate plans and forcing him into situation where he cannot refuse, and money is not really a thing for someone who meets old man's expectations, as there are many safer jobs than protecting woman in the midst of war.
How to convince this person?