Luke already foresaw through the Force how his conflict with Jabba was going to end, or at least what the ideal outcome for him and his friends was. He just wanted to give Jabba a chance to solve things nonviolently.

You can notice that Luke nods along and looks almost pleased when 3PO is announcing their death sentence from Jabba. At that point, Luke has exhausted every alternative option (bargaining, stealth, Force persuasion), and he knew beforehand that this was what was most likely to happen.