We don't know all of the details, but I think there are a couple of simple facts. 1. Mase has signed a legally binding contract with Bad Boy. 2. 50 Cent hasn't offered enough, in Diddy's eyes, to trade for Mase. So, with that in mind, I am going to simplify this argument a bit. Throw out anything you think about Diddy and 50. Clean slate. Pick a piece of property that you own that has some sort of value. Now, imagine this scenario:

Some Guy: "Hello, I want to buy that piece of property that you own. How much?"
You: "I want $500 for it."
Some Guy: "Well, I don't want to pay that. I'll give you $300."
You: "No, sorry. I want $500."
Some Guy: "Just give it to me for $300."
You: "No."
Some Guy: "Despite the fact that it is your property that you have a right to sell at the price of your choosing, I'm going to pretend that that isn't true and I'm recording a diss track."
You: "Have at it."

Some Guy is 50 Cent. You are Diddy. Mase's recording rights are a piece of property. If you still think Diddy should just let Mase go or go for whatever 50 Cent is offering... I'd like to visit your house and buy some things.