One option is to say something like "When (you get a 6 figure job, your options vest, ...), you can return the favor or pay it forward". Two birds with one stone: express confidence in the person and make it feel less awkward.
A lot of people (arguably wrongly) tie their self worth to their wealth. This can come off as "when you're worth as much as a human as I am!"
Or, for someone in a career where 6 figures do not happen, this is equivalent to telling them that nope, they can never repay this favor!
The only time I could see this working is if you had some celebration-worthy achievement which also had money associated with it. Then, throw a party to celebrate your achievement (say, the company you started went public) -- friends like to support those! -- and then it's socially acceptable that you're using the money from said achievement to fund the party.
But that's exactly what you want. You're trying to spend money on friends without making them feel obligated to spend money on you.
Perhaps there's better wording (and I'd like to hear it), but I think the best approach is rather than telling them that they have no obligation to return the favor, shift the obligation to something much less onerous for them by shifting it in time or in kind.
A good friend will believe you when you tell them there is no obligation to return the favor, but a more casual friend is less likely to believe you and more likely to think you're shifting the social obligation balance in your favor and might call on it in the future. Better to make it explicit to head that off.
Instead, suggest something they could do today which is of equal social cost to them and equal social benefit to you. Say -- if you paid for an expensive weekend vacation home, maybe next year they can host you at their Uncle Whoosit's place up in rural Maine.
When people say that they want to spend money on friends & family with no obligation in return, they're network building. That they'd expect the favor to be returned if the situation were reversed. Even if nobody really expects the situation to be reversed. I guess we could lean into that:
"When the zombie apocalypse happens, I expect you to have my back".
If I don't value the means by which someone could reciprocate a gift, I don't give the gift. And I would not feel that a gift was genuine or meaningful if someone gifted me something expressly for the purpose of "network building", which I am unable to reciprocate in a way they find acceptable. It creates an awkward power imbalance that I don't think belongs in a healthy friendship.
But what he said is straight-up a terrible idea.
If they do want to be in the deli job forever, then yah, my phrasing is awkward. There aren't too many people who want to work the deli forever, but there are some.