The Envelope, Let Me Receive It Once

Airi sees mysterious things invisible to others.
She, who was pressured by her family to marry quickly to carry on the lineage, makes a proposal to Duke Elkyeus.
“I want you to take me and say, ‘I won’t marry anyone else but this woman.’”
“Hmm, and?”
“Since I’m a commoner, surely they’ll oppose me. If I persistently refuse to break up, wouldn’t you give me an envelope to leave?”
“And then you’ll take the envelope and leave?”
“Yes. Then, you, Your Grace, will be protected after saying, ‘Didn’t I say I wouldn’t marry anyone but that woman?’”
The Duke no longer has to endure naggings telling him to marry, and he would be able to afford his grandmother’s medical expenses.
Airi thought it was a perfect plan. But then…
“All my recovery is thanks to you. I’ll repay this favor for sure.”
Even the dowager duchess who was bedridden for the whole 360 days a year due to illness.
“Airi, let’s draw and play together today too!”
Even the young lady who was so shy that she wouldn’t easily give her company to others.
“I will pledge allegiance to the future Duchess.”
“Future Duchess, please call me Grandpa once. You don’t need to be embarrassed. Come on.”
Even to the closest relatives of the duke’s family, the heads of the senior families.
“Fine, I concede. Let’s proceed with the engagement first.”
Even the dowager duchess, in the end, supports this marriage?
“Something terrible happened, Your Grace! We might actually end up getting married!”
“We can’t help it. Let’s start with the engagement.”
“What? How will we fix this mess?”
“Just trust me. Engagements are easier to break than marriages anyway.”
Can I really trust him? Can we really break off the engagement?
I just wanted to receive the envelope!