I thought this was revealing:
Two of their most prominent signings last year were Venezuelan outfielders Albert Guaimaro and Simon Muzziotti for $300,000 bonuses each on July 2. Guaimaro was BA’s No. 15 international prospect last year, while Muzziotti ranked No. 24. They have both been playing in the Dominican Summer League.
Seems the $300K limit isn't very binding, unless the Red Sox were making under the table payments.
Note that $300K is a 6th rd draft pick allocation.