There’s been broken promises, but it’s actually been mostly on the side of teams not keeping their promise of signing guys, rather than the other way around. This doesn’t tend to happen to the higher end guys though, like Manuel Beltre or Orelvis Martinez.
The whole system is pretty flawed and they’re gearing towards an international draft in the next CBA so everything could change. But to give a little context of how things generally work now..
Most of these kids being scouted are down south in dominican republic, venezuela, cuba, mexico, panama, etc.
They typically come from a very modest family, but not always (e.g. Vlad Jr), so when their kids show promise they get into amateur showcases and also join a baseball program guided by a trainer. These guys will help groom them and help promote them to teams while overseeing their development. They sometimes participate in amateur tournament too where teams will send international scouts. Of course they’ll usually be paid a commission from these kids signing bonuses.
At some point a team will approach the players and trainers and come to an agreement. This can be as early as 3 years before they’re even eligible for the IFA period, 13-15 year olds, sometimes earlier. So you can imagine their bodies will change drastically over the course of their development by the time they sign. But with how competitive international scouting is now, it’s getting earlier, it’s basically a race for teams to get to the best guys first. But sometimes its very difficult to predict as you’re projecting kids that haven’t finished puberty. They’ll come in agreement with dozens of guys hoping to land the next Guerrero, Soto, Acuna, Tatis, etc. a million dollars is literally generational life changing money for a lot of these kids families. A team will even supply these kids with equipment (bats, gloves, uniforms) for years as they grow and enter IFA eligibility. So you can imagine the amount of emotional investment on both sides to see a kid grow from 13-14 to when they finally sign and enter the farm system. Sometimes there’s multiple teams interested and get to a kid before they’re fully committed, it’s case by case. But usually no, when a high profile prospect like Beltre is already decked out and advertising the team’s logo, other teams know he’s already taken and won’t be breaking his commitment unless there’s special reasons. At this point they’ve likely already agreed to a lucrative $. By the time a kid is this committed, he won’t even do showcases anymore or workouts for other teams, so they have no idea of the prospects growth and won’t throw half their bonus pool for a kid to outbid the parent team. Funny things do happen but generally it doesn't work that way.
There’s a lot of these stories. Acuna was signed for only 100K with the Braves and they basically got to him really early and it was a lucky break for them, you can read about him here: https://www.google.ca/amp/s/www.mlb.com/amp/news/how-the-atlanta-braves-signed-ronald-acuna-jr-c283634544.html
Ohtani was a weird case, but essentially the Angels got to him very early as well. Although it wasn’t a commitment, Ohtani favored the Angel due to Billy Eppler when deciding who to sign with and it ultimately paid off for them.
https://theathletic.com/1880892/2020/06/25/inside-the-unheard-of-process-that-brought-shohei-ohtani-to-the-majors/