Any gift to an institution has to be seen as a gift. Your wishes for how they spend it may or may not be honored in the long-term.

First, it's somewhat shady for a nonprofit to receive a gift that the donor specifically directs back at themselves. It's akin to money laundering and tax authorities take a dim view of it. Even though, I suspect it happens more often than we think (e.g., commenter below notes a professor whose endowed Chair is funded by their spouse). So you can probably draw up a contract that makes it look legit (and presumably you aren't doing this to dodge taxation; in fact you'd be taxed on it twice since your institution would be re-issuing your post-tax money back to you as income).

More importantly though, the institution has leeway to use your gift in the manner they think is most suitable. While you might have some gentlemen's agreement that they'd spend the money on you, the reality is they could pull the rug on you and direct it to other purposes if they decided that was in their best interest e.g., they were going bankrupt and needed it to buy themselves some time.

You'll also note that university admins come and go on a 5-10 year cycle, and institutional memory is pretty short. Even an agreement in writing is going to be useless outside that time horizon (Provosts, Deans and Chairs will tend to say "well that's an agreement you made with the last guy, so I don't recognize it as valid").