Nice if it works out like that. I don't claim to understand how a draft pick (and agent) sees things. If an agreement is reached before the draft on a dollar figure, sure - that's surely how a lot of the mid-round picks did it, for draft certainty on both sides.
But say it's perceived as an N-player draft with a big dropoff in talent at player number N+1. Why would any of those top N players agree before the draft to under-slot with team N? Each player has their sights set on the very highest slots, and will be at least a little disappointed that they are the one that drops to N. I would think the pre-draft conversation will go, "I'll be glad if your team drafts me and I'll sign for slot, but truly I expect I'll be gone by the time it's your pick." They won't say, "well, if I drop to number N, I guess I'm not as good as I thought. I'll take less, and be grateful." Are they really thinking team N is going to draft player N+1 in this scenario (such as this year, where there wasn't even a consensus on who would be #6.) , just to save money so they can spend it on an even lesser high-school prospect (who isn't good enough apparently for anyone to draft high enough to entice them with what amounts to second-round money in the first place)?