I cant really blame Moises here. He is 21, grew up in a poor family as the youngest of 10 siblings, playing football in the streets with no real education. I bet he knows nothing about contracts and finances.
Young, uneducated people are easy to manipulate. Especially with money, especially when u have a poor family.
His agent probably told him: "cmon, sign this, we will both get a little bit of money here and in the summer u can leave, win win"
Typical greedy agents, and Brighton happy to srew him and exploit this.